Can Send As a different alias address in OWA, but can't in Outlook using Managed list?

Three separate things have to line up in Exchange Online before Outlook desktop will keep the alias:

1. The tenant setting: Exchange admin center → Settings → Mail flow → "Turn on sending from aliases" (Set-OrganizationConfig -SendFromAliasEnabled $true). Without it, Exchange Online itself rewrites the From to the primary address no matter what the client sends — which is why OWA "working" can be misleading if you only looked at the Sent Items copy. Microsoft's write-up: https://techcommunity.microsoft.com/blog/exchange/sending-from-email-aliases-%E2%80%93-public-preview/3070501
2. Outlook must send the alias as a plain SMTP address, not resolve it against a cached contact. Type it into From → Other E-mail Address rather than picking the autocomplete entry, and check the received message's headers, not Sent Items.
3. Older Outlook builds rewrote the alias in Online Mode; Microsoft fixed that in a 2024 build, so update the client.

Disclosure: I'm the founder of Servolutions and we make a sender-address add-in — but it targets Exchange on-premises, where no such tenant setting exists. For Exchange Online you don't need it; the three steps above are the whole fix.

Hi @Brad Wooder ,

Did you setting on the EXO mail flow settings sending from aliases?

Hope I was Help you

My regards

Paolo

Hi @Brad Wooder ,

Try to clear the autocomplete list: Clear the AutoComplete list Or Start Outlook by using the command: Select Start, and then select Run. Type Outlook.exe /CleanAutoCompleteCache.

Check this link: https://www.howto-outlook.com/faq/aliases.htm and test if it works. (Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

Make sure you're typing in the email alias in the From > Other e-mail address box, do not select any stored/autocomplete entry. I'm using an Outlook build that's on the semi-annual channel, and it works fine there.

Another thing to have in mind is that Outlook will resolve the recipient address on the received message against any existing contacts, and doesn't always indicate the alias used. The best way to confirm the address used is to check the From header of the message.