AutoMySQLBackup should be telling Mutt not to save any outbound messages. I found a 1.5GB "sent" file in "/root" which contains all of Mutts messages.
You seem to have CSS turned off.
Please don't fill out this field.
A bit more on this:
You should apply the -e "set nocopy" flag to fix this issue.
"This variable controls whether or not copies of your outgoing messages will be saved for later references."
In the context of being a backup script, the behaviour of saving sent messages is not needed.
Added in version 3.0_beta.