We used to have a distribution group that I deleted and reused the email addresses from that distribution group to create a mailbox. The mailbox cannot send out to anyone external but can send internal emails, they get an NDR and it fails to send.
I have checked the mail flow settings for that user and created another new test user and they can send fine externally, it's only the one account.
There must be something cached from the previous distribution groups. Does anyone have any ideas?
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may
be preventing delivery.
#550 5.7.1 You are not allowed to send external Email ##<o:p></o:p>