A couple of us have email accounts on a outlook.office.com via an Exchange Online Essentials subscription. I also have the admin login on admin.microsoft.com but try to avoid going anywhere near it.

We were recently migrated to new email addresses: we used to be on one of the old Demon ISP subdomain id@xxxx.demon.co.uk addresses but those email addresses - the last surviving relic of the Demon brand - have finally been laid to rest and we're now on a proper domain of our own choosing. This was all handled pretty smoothly via the hosting company who took over this part of the Demon brand. However there is one aspect of the migration which is still puzzling us.

When I use the outlook.office.com web interface to compose a new email, From: shows myid@newdomain as expected.

However when the other user uses it (on a different PC), their From: shows theirid@olddomain.demon.co.uk. Clicking the From: brings up a menu with theirid@newdomain allowing it to be selected, but there seems to be no way to get the From: to switch permanently for all new emails.

In practice it seems to make absolutely no difference whether the From: is changed or not: recipients always see it coming from theirid@newdomain . So maybe we shouldn't worry about this. However it's a bit disconcerting to see it, and the user is worried it will leak out somewhere and people will continue replying to the old address (which no longer works).

We tried clearing all outlook/office/microsoft-related browser cookies and local storage. It made no difference.

I logged into the admin pages. Both users look pretty identical. Primary email address is ...@newdomain, the old addresses are still listed as aliases (there was an overlap period when both worked). However there seems to be nothing to indicate that for one user but not the other the old address should still be used as the default from.

So I'm puzzled (as it seems are the domain hosting company who must have been prodding this stuff as part of the migration).

Any ideas?

Googling the topic leads to quite a few pages suggesting that the default From: can be changed from a user's own Outlook settings' page for "Sync email" (example). However the Sync page no longer has the setting mentioned, and I have an idea it disappeared from there when support for "connected accounts" was dropped a few years ago.

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