It appears that there is some version of the Dovecot LDA that is sending an RFC 3798 MDN rather than an RFC 3464 DSN to report non-delivery to a user who is over quota.
See <https://bugs.launchpad.net/mailman/+bug/693134> for a discussion of this and <https://bugs.launchpad.net/mailman/+bug/693134/+attachment/1773018/+files/Automatically%20rejected%20mail.msg> for the actual MDN message.
The issue is that apparently a user was over quota and Dovecot returned an MDN with a disposition of deleted. This is intended to mean the the mail was delivered to the user but the user deleted it without reading it. If the message was actually not delivered, this should be reported to the MTA so it can return an appropriate SMTP status, or the LDA should send an RFC 3464 DSN with an action of failed.
Note that I do not actually use the Dovecot LDA nor do I have any access to the system where the notice in the bug report was received, so I may have incomplete information and this may not be relevant to current versions of Dovecot, but I think it is wrong for Dovecot to report over quota non-delivery via an MDN.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan