It doesn’t appear to be an Outlook issue or anything else other than simply the mailbox that Avast has set up to process the reporting of the bad Phishing is full:
Hi. This is the qmail-send program at mailshell.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
dev80-alwil-missed.spam@dev80.mailshell.com:
MessageID 001801ce4cc4$0e502f00$2af08d00$@gmail.com, XJUNK1 0
user.215933.missed_unprocessed: Over quota missed.spam@alwil.mailshell.com is over quota.
They need to clear the box or TELL US HOW TO TURN OFF THE REPORTING. This must be stored in the registry somewhere or other file. I don’t see a setting in the maintenance control panel.