Definitely could be a mail sending problem if your SQL dump too large
mysqldump -ce my_db --user=my_user --password=mypass |gzip > ~/DBBackup.`date +\%F`.gzip; uuencode ~/DBBackup.`date +\%F`.gzip DBBackup.`date +\%F`.gzip |mail -s "my DB" [email protected]; rm -f ~/DBBackup.`date +\%F`.gzip