On 2024-01-14 15:28, Alexander Huynh via mailop wrote:
From a spam point of view, signing up for a domain is a barrier of
entry which some may consider too much trouble.
This may play into why there's a larger distribution of unwanted mail
on the freely-provided `*.onmicrosoft.com` subdomains.
It may be useful to add that I administer a MS365 tenant, having
migrated to it recently at $DAYJOB. I don't consider myself particularly
skilled, so consider this a very "low-hanging-fruit" assessment.
These are the situations where you'll need the onmicrosoft moniker:
1) Brand new tenant, as others have mentioned. It starts with
account@<something>.onmicrosoft.com. You're supposed to set up your
domain and take it from there.
2) Forward e-mail from on-premise to cloud. When your domain points to
your on-premise SMTP, you can then relay it to
<something>.mail.onmicrosoft.com. Notice that it then generates the same
identifier <something> under .*mail*.onmicrosoft.com. As far as I
recall, this one is not even set up for outbound e-mail, just for inbound.
3) You *can* disable outbound for .onmicrosoft.com, and you should after
you set it up - for example, our organization has bilingual domains, and
as soon as I enable the option for our members to pick the domain when
sending through Outlook on the Web, onmicrosoft also showed up there.
Confusing and unnecessary.
4) You can NOT remove the onmicrosoft.com domain from the account, nor
you can change the one you pick when creating the tenant. It is an
internal reference, but that's it.
This is all to say: there's no valid reason I can see for anyone to use
their onmicrosoft.com domain for outbound e-mail. Even if you're
relaying, you'll use <tenantid>.mail.onmicrosoft.com, and that's inbound
only.
Spammers rely on the ease of creating a 365 trial account, and
.onmicrosoft.com being there and ready for action, and the fact that all
e-mail admins hesitate to block the big providers.
As a result, thanks to this discussion, that'll be my first thing to do
on Monday.
Kind regards,
Alberto Abrao
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