Users removed from DDL giving NDR

[spiceuser-phdec]

We have removed some users without mailbox from DDL, using recipient filter. It is an on prem DDL.
When user is sending email to DDL, he is getting NDR.
The NDR says that 554 5.4.12 : possible mail loop detected.
Probably because we have internal relay and the mail is looking for the mailbox in exchange on prem.
But why is the email still going to the user when he is not a part of the DDL ? Its happening with a lot of users removed from DDL.
I have extracted the DDL member list and verified that these users are not part of it.

And is the error **554 5.4.12 ** because of Internal Relay setting on the domain ?

[Rod-IT]

Unless the email is being sent via an app, it’s not likely touching your internal relay.

If you are hybrid it may be sending it from on-prem to 365, then 365 back to on-prem because it can’t find the mailbox in either place.

Those users without mailboxes will have bounced from day 1, however if you’ve only just removed them, you need to give the OAB time to create a new one and users to sync with it, at the moment users will be emailing the DDL based on cached information.

If they email via OWA or outlook (non-cached) they wont have this issue

If you can confirm the above, there may be more to check.

@spiceuser-phdec

why is the email still going to the user when he is not a part of the DDL

Cached data on the senders side, have them click the X next to the DDL as they start to type it, then pick it manually from the GAL the first time they email it. They will be using the cached copy, which still has the users in it.

[spiceuser-phdec]

The members were removed more than a month ago. Email was sent from Outlook. I will ask to remove the auto fill in Outlook and type the ddl manually.

[Rod-IT]

That is likely the issue in that case, autofill keeps cached copies, those are not always the latest.

[afroz-stellar]

The issue is likely caused by Outlook’s autofill using an old cached version of the DDL that still includes removed users. This can cause mail to route incorrectly and result in the 554 5.4.12 loop error, especially in an Internal Relay setup.
Ask the sender to remove the DDL from autofill (click the “X” next to it when typing), then select the DDL fresh from the address book. This should stop the issue.