User getting "invalid emails" for many known contacts - but address works fine for other users

[kennysarmy]

We have a user who is getting warnings pop up in Outlook (OWA) that email addresses are invalid.

 

If I copy and paste the exact same email address in to my TO: line they are all totally fine.

 

Any ideas? Very odd one!

[NegativeKillDeath]

Internal or external domain? Are they relying on autocompleted to fill the email address? I would typically delete the suggestion and retype the whole address again.  

[kennysarmy]

4 minutes ago, NegativeKillDeath said:

Internal or external domain? Are they relying on autocompleted to fill the email address? I would typically delete the suggestion and retype the whole address again.  

 

No matter how it's populated in the TO: box it says it's invalid.

 

If I type in an invalid email such as blah@blah it doesn't get reported haha

 

 

[kennysarmy]

The user has an existing distribution list that used to work fine, but now fails on 13 addresses out of 22!

 

 

Edited November 7, 2025 by kennysarmy

[chaplic]

Do they have that person as a contact in their personal address book, which also has an old x400/x500 address associated with it? nuke the contact 

[kennysarmy]

On 07/11/2025 at 16:16, chaplic said:

Do they have that person as a contact in their personal address book, which also has an old x400/x500 address associated with it? nuke the contact 

 

Thanks, done this, deleted one of the contacts from their address book, recreated from scratch, same issue.

[chaplic]

why do they need a personal contact at all? delete it, and in the to: field  start to type their email then when the system suggests the address, press the X at the end of it to delete the address and re-type it.

[kennysarmy]

It really does not matter how the addresses get populated.

 

Email addresses which work totally fine for other users are just being rejected for this user, and it's not device specific.

 

MS are dragging their heels organising a remote session so I can demonstrate the issue, meanwhile the user is unable to email at least 12 emails that we've identified as being rejected.

 

It's very odd.

[chaplic]

if they have a stale entry for the user with an invalid x400: address present, it absolutely does matter and would cause this behaviour. I can't think of any other cause, but all the best with MS.