Why are my colleagues email addresses coming up as invalid?

BRENNAN, Kate (LEEDS TEACHING HOSPITALS NHS TRUST) 35 Reputation points
2025-08-27T15:10:03.3266667+00:00

When I email some colleagues, I get the message that their email addresses are invalid. The email addresses are not incorrect and have not changed in the years that I have worked here. They are still receiving emails to that address but I am unable to send them any. This is also the case for a few other colleagues in my department i.e., they also get the error message about some colleagues email addresses when they try and send them emails.

Outlook | Web | Outlook.com | Email
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Arlene D 36,225 Reputation points Independent Advisor
2025-08-29T06:21:49.5733333+00:00

Got it. Thanks for confirming, Kate. Since you only use Outlook on the web, clearing a local cache is not needed. If the error appears there but messages still go through, the issue is likely with your organization’s address directory. In this case, I recommend asking your IT admin to review and refresh the Global Address List in Exchange Admin Center. That ensures Outlook on the web is pulling the correct and current address data. Just let me know if the issue still persists after asking your IT.

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  1. JeGO_BE 21 Reputation points
    2025-08-29T07:13:34.9366667+00:00

    It seems the issue here was solved automatically here. Probably after a refresh/download in the background of the GAL done by Microsoft Office 365 automaticaly.

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  2. JeGO_BE 21 Reputation points
    2025-08-28T14:12:23.0033333+00:00

    I think we have for very limited amount of mailboxes the same issue.
    Especially when we expand a contact list from personal mailbox, the contacts show up as invalid in outlook web. Outlook Desktop doesnt have this issue.

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  3. Arlene D 36,225 Reputation points Independent Advisor
    2025-08-27T18:23:26.2733333+00:00

    Oh, I see. thanks for clarifying. Since your colleagues still receive the emails, the addresses are valid. The error usually comes from Outlook using old cached data or a delay with the address book. I recommend checking if the same issue happens in Outlook on the web. If it only happens in desktop Outlook, clear the auto-complete cache and update Outlook to the latest version. If it still shows, your IT admin should refresh the Global Address List to make sure Outlook is in sync.

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  4. Arlene D 36,225 Reputation points Independent Advisor
    2025-08-27T15:49:01.6833333+00:00

    Hello. This error often happens when Outlook uses old cached contact details. I would recommend removing the cached entry and selecting the colleague’s address from the Global Address List instead.

    This ensures you are using the current address stored in your organization’s directory. If the email works from the Global Address List but fails with auto-complete, clearing the Outlook cache should fix it.

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