Someone in your organisation changes their email address.

Perhaps:

jane@oldcompany.co.uk

becomes:

jane@newcompany.co.uk

Microsoft 365 is updated correctly.

The Global Address List shows the new information.

Yet when somebody starts typing Jane into Outlook, the old address still appears.

Worse, Outlook may automatically select it.

Messages might:

• go to an old address
• generate a non-delivery report
• resolve to the wrong recipient
• repeatedly suggest a contact who left years ago

The obvious conclusion is:

“Microsoft 365 hasn't updated.”

Often, Microsoft 365 is completely correct.

The stale information is somewhere else.

The fastest way to fix it is to identify which source is providing the old address.

Why Outlook Can Show an Address That No Longer Exists

There isn't one single Outlook address database.

Depending on the Outlook version and where the recipient appears, information can come from:

• Exchange Online
• Global Address List
• Offline Address Book
• AutoComplete
• Microsoft Search
• Outlook Contacts
• personal contacts
• previous recipient information

That explains one of the most frustrating situations:

Microsoft 365 Admin Centre shows the correct address

but:

Outlook still suggests the old one.

Both can be true.

First: Test Outlook on the Web

This is one of the quickest diagnostic tests.

Open Outlook on the web and create a new email.

Start typing the person's name.

Outlook on the web shows the correct address

The Microsoft 365 directory is probably correct.

Concentrate on the desktop Outlook client, its local data or its recipient suggestions.

Outlook on the web also shows the wrong address

The problem may be:

• Exchange Online recipient information
• Microsoft Search
• a saved contact
• another directory object

Do not start rebuilding the desktop Outlook profile yet.

You have evidence that the problem is not confined to that computer.

Check the Actual Microsoft 365 Recipient

For an internal employee, an administrator should verify the user's Exchange Online information.

Check:

• primary email address
• aliases
• display name
• username
• proxy addresses

An old address remaining as an alias is not automatically wrong.

Keeping an old email address as an alias can be intentional so messages sent to the previous address still arrive.

The important question is whether:

the old address is merely retained for delivery

or:

Microsoft 365 still considers it the wrong primary identity.

Those are very different problems.

The Most Common Cause: Outlook AutoComplete

Outlook remembers recipients you have previously emailed.

Microsoft calls this the AutoComplete list, historically also known as the nickname cache. It can contain display names, SMTP addresses and legacy Exchange addressing information.

That is why Outlook can remember an old address long after an administrator changed the Microsoft 365 account.

Imagine:

Jane Smith

previously used:

jane@oldcompany.co.uk

Her account now uses:

jane@newcompany.co.uk

You type:

Jane

and Outlook offers the old entry.

That does not necessarily mean the Global Address List is wrong.

Outlook may simply remember that you previously emailed the old address.

Fix One Incorrect AutoComplete Entry First

Do not immediately wipe the entire AutoComplete list.

Open a new message.

Start typing the recipient's name.

When the incorrect suggestion appears, highlight it and select the X where available, or press Delete.

Microsoft specifically recommends removing individual unwanted AutoComplete entries this way.

Then:

1. Open the address book or directory.
2. Select the correct recipient.
3. Send a new message.
4. Test AutoComplete again.

This is much less disruptive than deleting every remembered recipient.

Why the Wrong Suggestion May Come Back

This is where modern Microsoft 365 makes the problem slightly more complicated.

In current Outlook for Microsoft 365 builds connected to Exchange Online, recipient suggestions can also be powered by Microsoft Search rather than exclusively by the traditional AutoComplete cache.

So if you remove an old AutoComplete entry and still see unexpected suggestions, do not assume:

“The cache didn't clear.”

The suggestion may be coming from another Microsoft 365 search source.

That is also why troubleshooting instructions written for Outlook 2007 or Outlook 2010 are increasingly unreliable for current Microsoft 365 environments.

Check Outlook Contacts

Search:

People / Contacts

for the affected person.

You may discover a manually saved contact containing:

jane@oldcompany.co.uk

while the Microsoft 365 directory correctly contains:

jane@newcompany.co.uk

Update or remove the obsolete personal contact.

This is especially common when:

• somebody changed employer
• a supplier changed domain
• an employee married and changed name/address
• two contacts have similar names
• an old contact was imported years ago

Check the Global Address List

If the wrong information appears in the organisation's address book rather than only when composing a message, investigate the directory itself.

For an internal Microsoft 365 user, verify that:

• display name is correct
• primary SMTP address is correct
• required aliases are correct
• there isn't a duplicate object
• an old mail contact does not still exist

Duplicate objects can be particularly confusing.

For example, Microsoft 365 may contain:

Jane Smith — User Mailbox

and:

Jane Smith — Mail Contact

with different addresses.

The user sees two Janes and picks the wrong one.

That is not an AutoComplete problem.

Clean up the directory object.

Cached Outlook Can Have an Offline Address Book

Classic Outlook using Cached Exchange Mode can maintain local mailbox data and an Offline Address Book (OAB).

Microsoft recommends Cached Exchange Mode for Microsoft 365 accounts, and the OAB allows address-book information to remain available even without a continuous Exchange connection.

That creates another possibility:

Exchange Online is correct

but:

the local address book has not caught up yet.

Download a Fresh Address Book

In classic Outlook, where the Offline Address Book is applicable, try:

File > Account Settings > Download Address Book

You can normally choose to download changes or obtain a fuller refresh.

Microsoft documents this as the mechanism for downloading the Exchange Online Offline Address Book.

Then restart Outlook and check the recipient again.

Remember that Exchange Online's OAB is generated periodically rather than instantly; Microsoft currently documents generation approximately every eight hours.

So a directory change does not necessarily appear in every cached Outlook client immediately.

Don't Confuse AutoComplete With the Address Book

This is the distinction that should sit at the centre of the article.

Wrong address appears only while typing To/Cc/Bcc

Investigate:

AutoComplete / Microsoft Search / Contacts

Wrong address appears in the company address book

Investigate:

Exchange Online directory / Offline Address Book

Outlook on the web is correct but one PC is wrong

Investigate:

desktop Outlook

Everybody sees the same incorrect recipient

Investigate:

Microsoft 365 directory

This simple classification can save a huge amount of unnecessary troubleshooting.

What If Only One Employee Has the Problem?

Suppose:

19 employees see Jane's correct address

but:

one employee still sees the old one.

That strongly suggests a user-specific problem.

Start with:

1. AutoComplete entry
2. saved Contacts
3. local Outlook/OAB data

Do not immediately modify Jane's Microsoft 365 account when virtually everybody else sees it correctly.

What If Everyone Has the Problem?

Now the investigation changes.

If multiple employees and Outlook on the web all show incorrect information, check:

• Exchange Online recipient
• aliases
• mail contacts
• groups
• duplicate directory objects

The probability of twenty independent Outlook caches all becoming wrong in exactly the same way is considerably lower.

Start centrally.

Old Addresses After a Mailbox Migration

Address problems become particularly interesting after:

• Microsoft 365 migration
• tenant migration
• hybrid Exchange migration
• mailbox deletion and recreation

Outlook recipient history can contain more than the visible SMTP address.

Microsoft documents that AutoComplete entries can include LegacyExchangeDN information as well as SMTP addresses.

That matters because Outlook may remember the identity of the old mailbox object, not merely:

person@example.com

This can produce non-delivery reports after a mailbox has been recreated even when the visible email address looks identical.

The IMCEAEX / LegacyExchangeDN Problem

A classic symptom after recreating or migrating an Exchange mailbox is an NDR containing something resembling:

IMCEAEX-_O=...

That strongly suggests an old Exchange legacy address is involved.

In those situations, deleting the stale AutoComplete entry can solve the individual user's problem.

But if many people are affected, an Exchange administrator may need to preserve the old LegacyExchangeDN as an X500 proxy address on the replacement object.

That allows old cached Exchange recipient references to resolve to the new mailbox.

This is particularly relevant after:

• mailbox recreation
• cross-tenant migration
• hybrid migration
• distribution-group recreation

Do not repeatedly clear everyone's Outlook cache when the better solution is repairing recipient compatibility centrally.

Clearing the Entire AutoComplete List

If AutoComplete itself is genuinely corrupted or full of obsolete information, classic Outlook provides:

File > Options > Mail > Empty Auto-Complete List

Microsoft also documents:

Outlook.exe /CleanAutoCompleteCache

as another supported method.

But understand the consequence:

you lose the remembered AutoComplete entries.

That is why I would not make this Step 1.

Remove the individual bad recipient first.

Don't Start Deleting RoamCache Files

Older troubleshooting guides often jump immediately into:

%LOCALAPPDATA%\Microsoft\Outlook\RoamCache

and tell users to delete files.

That should not be the first response to one stale email address.

Current Outlook/Microsoft 365 recipient suggestions are more complicated than a single local cache file, and Microsoft documents supported UI methods for removing individual AutoComplete entries.

Start with the supported, least-destructive method.

Don't Rebuild the Outlook Profile First Either

Creating a new Outlook profile can solve certain Outlook problems.

But it is excessive when:

one recipient's old address appears in AutoComplete.

A better escalation order is:

remove bad suggestion → check Contacts → test Outlook on the web → verify directory → refresh OAB → investigate profile

Rebuilding Outlook should come considerably later.

A Useful Example

Jane's email changes from:

jane@oldcompany.co.uk

to:

jane@newcompany.co.uk

Microsoft 365 Admin Centre shows the new address.

Outlook on the web shows the new address.

Twenty employees see the new address.

One employee's desktop Outlook suggests the old address.

That is overwhelmingly pointing towards that employee's:

AutoComplete / Contacts / local Outlook data.

Delete the stale suggestion.

Select Jane from the correct directory.

Send a new message.

Problem solved.

Another Example

A mailbox is deleted and recreated.

The new mailbox has exactly the same visible SMTP address.

Several employees receive an NDR containing:

IMCEAEX

after selecting the person from previous recipient history.

Now the problem is not simply:

“Outlook remembered the wrong email address.”

It may be remembering the old Exchange object's LegacyExchangeDN.

That should prompt an administrator to investigate the appropriate X500 address rather than clearing caches across the entire organisation.

Shared Mailboxes and Distribution Lists Can Have the Same Problem

The same principles apply to:

• shared mailboxes
• distribution groups
• Microsoft 365 Groups
• mail contacts
• room mailboxes

If an object has been:

• renamed
• deleted
• recreated
• migrated

users may retain stale recipient information.

Again, determine whether the problem is:

one user

or:

everyone.

That tells you where to investigate first.

The Fast Troubleshooting Flow

If Outlook keeps showing an old email address:

1. Test the recipient in Outlook on the web.
2. Check whether the problem affects one user or everyone.
3. Remove the individual stale AutoComplete suggestion.
4. Select the correct recipient from the directory and send again.
5. Check personal Outlook Contacts.
6. Verify the recipient in Microsoft 365/Exchange Online.
7. Check for duplicate users, contacts or groups.
8. Refresh the Offline Address Book where applicable.
9. If there is an IMCEAEX NDR after migration/recreation, investigate LegacyExchangeDN/X500.
10. Clear the entire AutoComplete list only if there is evidence the broader cache is corrupted.
11. Rebuild the Outlook profile only after simpler causes have been eliminated.

The key principle is:

Find out where Outlook obtained the old address before trying to clear everything that might contain it.

How Hamilton Group Can Help

Outdated Outlook addresses can be simple AutoComplete problems, but they can also reveal issues with:

• Exchange Online
• Microsoft 365
• mailbox migrations
• Outlook profiles
• Offline Address Books
• shared mailboxes
• distribution groups
• LegacyExchangeDN/X500 addresses
• duplicate directory objects

Hamilton Group can diagnose the problem at the correct layer rather than repeatedly clearing Outlook caches and hoping the old address disappears.

For businesses, we can also help with:

• Microsoft 365 administration
• Exchange Online
• Outlook support
• mailbox migrations
• Microsoft 365 tenant management
• email security
• user onboarding and offboarding

Visit hgmssp.com or call 0330 043 0069 to discuss Microsoft 365 and Outlook support.