Your Email Program Is Using Outdated Address Information in Microsoft 365

You start typing a colleague’s name into a new email, and Outlook suggests an address that should no longer exist.

Perhaps the employee changed their surname, moved to another department, received a new email address or left the business months ago. The Microsoft 365 admin centre shows the correct details, yet Outlook continues displaying the old information.

This can be confusing, particularly when the wrong suggestion looks almost identical to the correct one. It can also cause failed deliveries, messages being sent to obsolete addresses or confidential information going to the wrong recipient.

The good news is that the problem is usually not with Microsoft 365 itself. More often, Outlook is retrieving information from an old AutoComplete entry, a personal contact, an offline address book or locally cached data.

This guide explains where outdated address information comes from, how to identify the source and how to correct it safely.

Why Outlook can show old email address information

Microsoft 365 can provide recipient information from several different places. These sources are not always updated at exactly the same time.

When you begin entering a recipient in the To, Cc or Bcc field, Outlook may search or display information from:

• Your Outlook AutoComplete list
• Personal contacts
• The organisation’s Global Address List
• The Offline Address Book
• Previous email conversations
• Suggested contacts and recent recipients
• Contacts synchronised from a phone or another account
• A shared mailbox or shared contact folder

This means the address shown in Outlook is not necessarily being retrieved directly from the current Microsoft 365 directory.

The most common cause is the Outlook AutoComplete list, sometimes called the nickname cache. Outlook builds this list automatically from people you have previously emailed. It can contain display names, SMTP addresses and older Exchange addressing information.

If a recipient’s details change later, the saved suggestion can remain in the list.

What the problem usually looks like

Outdated address information can appear in several ways.

You might notice that:

• Outlook suggests a former employee.
• A colleague’s old surname still appears.
• The display name is correct, but the email address is wrong.
• Outlook selects a personal address instead of a business address.
• A message generates a non-delivery report.
• Replies fail even though newly composed messages work.
• Outlook on the web shows the right address, but desktop Outlook does not.
• One computer shows old details while other devices are correct.
• The Global Address List is correct, but AutoComplete still presents an obsolete entry.
• A removed suggestion returns later.

The precise behaviour helps identify where the outdated information is being stored.

First, check whether Microsoft 365 has the correct details

Before clearing Outlook data, verify that the organisation’s directory information is correct.

An administrator should check the user in the Microsoft 365 admin centre and, where appropriate, the Exchange admin centre.

Confirm the following:

• Primary email address
• Username and sign-in name
• Display name
• Email aliases
• Proxy addresses
• Mailbox type
• Whether an old contact or guest account still exists
• Whether there are duplicate users, contacts or mail-enabled objects

It is possible to update someone’s primary address while leaving an old alias in place intentionally. This is often useful because it allows messages sent to the previous address to continue arriving.

However, duplicate directory objects can create confusing search results. For example, Microsoft 365 might contain an active user, an old mail contact and a guest account for the same person.

Do not remove an alias or directory object until you understand why it exists. Removing an old address prematurely could cause legitimate messages to bounce.

Remove an individual incorrect AutoComplete entry

When the problem affects only one recipient, removing that specific suggestion is usually the best first step.

In Outlook:

1. Create a new email.
2. Begin typing the person’s name or address.
3. Wait for the incorrect suggestion to appear.
4. Highlight the unwanted entry.
5. Select the X beside it, where available.
6. Enter the correct address manually.
7. Send a new message to rebuild the suggestion.

Microsoft confirms that Outlook’s suggested-recipient list grows as new recipients are emailed. Removing the obsolete suggestion and sending to the correct address allows Outlook to learn the updated entry.

Make sure you type the full correct address rather than immediately selecting another suggestion that may also be outdated.

Be aware of a current AutoComplete issue

Microsoft has documented a problem affecting some Exchange Online users where the X button appears to remove an AutoComplete recipient, but the suggestion later returns.

Microsoft states that traditional steps such as disabling AutoComplete or clearing the cache may not permanently resolve this particular behaviour while the issue remains under investigation.

If an entry repeatedly returns after removal, test the same address in Outlook on the web and check whether the information exists in personal contacts, recent recipients or the Global Address List.

This is important because repeatedly clearing the desktop cache may not help when the suggestion is being synchronised from another Microsoft 365 source.

Check personal contacts

A personal Outlook contact can override or compete with the organisation’s directory entry.

Open the People or Contacts area in Outlook and search for the recipient using:

• Their name
• Their current email address
• Their previous email address
• Their company name
• Their telephone number

You may find more than one contact for the same person.

Delete obsolete contacts or update them with the correct information. Also check contact folders connected to other accounts, such as Gmail, iCloud or an old Exchange mailbox.

On mobile devices, Outlook may also have permission to access the phone’s native contacts. An old entry stored on the device can therefore reappear as a suggestion even after the Microsoft 365 contact has been corrected.

Check Outlook on the web

Outlook on the web is one of the most useful diagnostic tools because it helps separate a local Outlook issue from a wider Microsoft 365 problem.

Sign in to Microsoft 365 through a browser, open Outlook and start a new email.

Search for the affected recipient.

There are two likely outcomes.

Outlook on the web shows the correct address

This suggests the Microsoft 365 directory is probably correct and that the desktop application has retained old information locally.

Focus on:

• AutoComplete
• Personal contacts
• The Offline Address Book
• The Outlook profile
• Cached Exchange Mode

Outlook on the web also shows the wrong address

This indicates that the information may be stored in Microsoft 365 rather than only on the computer.

Check:

• Directory users
• Mail contacts
• Guest accounts
• Suggested recipients
• Shared contacts
• Duplicate objects
• Email aliases

This comparison can prevent unnecessary rebuilding of Outlook when the actual issue exists in the cloud directory.

Update the Offline Address Book

Classic Outlook for Windows commonly uses Cached Exchange Mode. This stores mailbox data and a local copy of address-book information so Outlook can remain responsive and continue working when the connection is poor or unavailable.

If a colleague’s details were recently changed, desktop Outlook may still be using an older copy of the Offline Address Book.

To download it manually in classic Outlook:

1. Open Outlook.
2. Select File.
3. Select Account Settings.
4. Select Download Address Book.
5. Clear Download changes since last Send/Receive when you want a full download.
6. Select the relevant address book.
7. Start the download.
8. Restart Outlook after it finishes.

Microsoft documents this process for manually downloading the Offline Address Book in Exchange Online. Clearing the incremental-download option requests a complete copy rather than only recent changes.

Allow time for Microsoft 365 changes to propagate before assuming that the update has failed.

Clear the complete AutoComplete list only when necessary

Classic Outlook includes an option to empty the entire AutoComplete list.

You can find it under:

File > Options > Mail > Send messages > Empty Auto-Complete List

This removes all saved recipient suggestions, not just the incorrect one.

Afterwards, Outlook gradually rebuilds the list as you send messages.

This can be helpful when:

• Many suggestions are wrong.
• AutoComplete appears corrupted.
• Old addresses repeatedly cause failed deliveries.
• The list contains years of obsolete recipients.
• Removing individual entries has not worked.

However, it is more disruptive than deleting one entry. Users may temporarily lose convenient suggestions for customers, suppliers and colleagues until the list is rebuilt.

Classic Outlook can also be launched with the following command-line switch:

outlook.exe /cleanautocompletecache

Microsoft documents this switch as removing all names and email addresses from the AutoComplete list.

Use this only after closing Outlook and confirming that the user understands the effect.

Why replies can still use an old address

Sometimes a newly composed message works, but replying to an old conversation fails.

That is because the original message may contain historic addressing data. Outlook can reuse that stored recipient information when replying or forwarding.

This is particularly common after:

• A mailbox migration
• A tenant migration
• A change from on-premises Exchange to Microsoft 365
• Deleting and recreating a mailbox
• Changing a recipient’s primary address
• Removing an old Exchange object

Try creating a completely new email instead of replying to the old thread.

Delete the recipient from the To field, type the current address manually and select the correct directory entry.

Microsoft has previously recommended deleting the affected AutoComplete entry when replies or forwards generate non-delivery reports linked to stale recipient information.

Check for duplicate contacts, guests and mail users

Microsoft 365 environments often accumulate duplicate identities over time.

A person might appear as:

• A licensed user
• A shared mailbox
• A mail contact
• A mail user
• A guest account
• A personal Outlook contact
• A Teams external contact

Each object may use a different address or display name.

This commonly happens when a contractor becomes an employee, a company changes domains or two Microsoft 365 tenants begin collaborating.

Administrators should search Microsoft Entra ID, the Microsoft 365 admin centre and Exchange recipients for the person’s old and new details.

Do not simply delete every duplicate-looking object. Confirm which applications, permissions, Teams memberships or shared resources depend on each identity first.

Shared mailboxes can have their own contact issues

When users send email from a shared mailbox, recipient suggestions may not behave exactly like those attached to their primary mailbox.

Shared mailboxes can have separate contacts, historic recipients and delegated-user behaviour. A suggestion may therefore appear only when sending from a specific mailbox.

Test the address while sending from:

• The user’s own mailbox
• The shared mailbox
• Outlook on the web
• A different authorised user’s account

This will help establish whether the problem follows the user, the device or the shared mailbox.

Outlook for Mac troubleshooting

On a Mac, compare the result with Outlook on the web first.

If Microsoft 365 contains the correct information but Outlook for Mac remains outdated, reset the affected account’s local cache.

In supported versions of Outlook for Mac, this can be done through:

Tools > Accounts > select the account > Manage > Reset Account

Microsoft explains that resetting an Outlook for Mac account clears downloaded data and downloads it again from the Exchange server.

Make sure Outlook is connected and fully synchronised before performing this step.

A reset can take time on a large mailbox and should not be the first response to one incorrect address suggestion.

When to create a new Outlook profile

If incorrect address information continues appearing after contacts, AutoComplete and the Offline Address Book have been checked, the Outlook profile itself may be damaged.

Creating a new profile can rebuild:

• Mailbox cache
• Address-book data
• Account configuration
• Search integration
• Local Outlook settings

This is a more substantial troubleshooting step and should usually be completed by an IT administrator.

Before removing the old profile, verify whether it contains:

• Local PST files
• Unsynchronised folders
• Personal signatures
• Additional mailboxes
• Archived data
• Custom settings

A new profile should not be used as a substitute for checking the Microsoft 365 directory. If the source information is wrong in the cloud, rebuilding Outlook will simply download the same incorrect data again.

How businesses can prevent stale address problems

Good account-management processes reduce the likelihood of old recipient details remaining across Microsoft 365.

When an employee changes their name, role or address:

1. Update the Microsoft 365 user.
2. Confirm the primary email address.
3. Decide whether the previous address should remain as an alias.
4. Check the display name and contact details.
5. Review Teams and SharePoint references.
6. Remove obsolete mail contacts and guest accounts where appropriate.
7. Inform regular correspondents of the change.
8. Ask users to remove old AutoComplete suggestions.
9. Test Outlook desktop, Outlook on the web and mobile access.
10. Document the change.

When an employee leaves, follow a structured offboarding process rather than simply deleting the account immediately.

A proper offboarding checklist should cover mailbox access, forwarding, aliases, shared files, mobile devices, sessions, licences and directory visibility.

Avoid sending sensitive information to a suggested address without checking it

AutoComplete is convenient, but it can encourage users to send email without carefully checking the selected recipient.

Before sending confidential or sensitive information:

• Expand the recipient card.
• Confirm the full email address.
• Check whether the recipient is internal or external.
• Be cautious when two people have similar names.
• Remove unexpected suggestions.
• Use sensitivity labels or data-loss-prevention controls where available.
• Consider disabling external AutoComplete suggestions in higher-risk environments.

The display name alone is not enough. Two recipient entries can show the same name while pointing to entirely different email addresses.

When to contact your IT support provider

Professional support may be needed when:

• The wrong address appears for multiple users.
• Changes in Microsoft 365 are not reaching Outlook.
• Messages are generating non-delivery reports.
• Old Exchange addresses remain after a migration.
• Duplicate directory identities exist.
• Shared mailboxes are affected.
• AutoComplete entries repeatedly return.
• The problem involves sensitive or misdirected email.
• Rebuilding the Outlook profile may be necessary.

The underlying cause could be a simple cached suggestion, but it could also involve directory synchronisation, Exchange configuration, identity duplication or an incomplete migration.

Final thoughts

When Outlook uses outdated address information, the visible suggestion is only the symptom.

The old data may be coming from AutoComplete, personal contacts, the Offline Address Book, an old conversation, a mobile device or a duplicate Microsoft 365 identity.

Start with the least disruptive fix:

1. Verify the correct information in Microsoft 365.
2. Remove the individual AutoComplete entry.
3. Check personal contacts.
4. Compare Outlook with Outlook on the web.
5. Refresh the Offline Address Book.
6. Clear broader caches only when necessary.
7. Escalate to profile or directory troubleshooting if the issue remains.

Taking a structured approach avoids unnecessary resets and reduces the risk of deleting legitimate aliases or contact information.

Hamilton Group provides Microsoft 365 support, Exchange Online administration and proactive managed IT services for businesses that need reliable email and identity management.

For help resolving Outlook or Microsoft 365 address-book problems, call 0330 043 0069 or visit hgmssp.com to speak with one of our experts.