Nice to see another Exchange bug (not confirmed with Microsoft) in practice. An user could not send message and accept invitations only with two internal recipients (all users from Exchange 2010 SP1). All mentioned messages were bounced with NDR:
5.7.1 Submission has been disabled for this account
It seems to be known issue reported for example by Nuno Mota here. Thx for that!
Basically, it is caused by value 0 KB applied on mailbox quotas despite of using the database quota defaults.
ProhibitSendQuota : 0 B (0 bytes) ProhibitSendReceiveQuota : 0 B (0 bytes) IssueWarningQuota : 0 B (0 bytes) UseDatabaseQuotaDefaults : True
The correction is simple and can be achieved by one cmdlet:
Set-Mailbox alias -IssueWarningQuota unlimited -ProhibitSendQuota unlimited -ProhibitSendReceiveQuota unlimited