No idea yet, sorry.
iRedMail uses full email address as login username. Although it's able to use username part in email address as login username too, but it's just for one domain.

we were planning on doing this, but later abandoned it. The plan was to allow people to login with their email address OR the username they had from the old platform, we had those usernames in a separate column in the database called modusname. As i recall the tricky bit was getting dovecot to look at both columns. Here's the modified dovecot conf I have in my notes, I'm pretty sure this works

iterate_query = SELECT username AS user FROM mailbox
password_query = SELECT password FROM mailbox WHERE username='%u'  OR modusname='%u'  AND active='1'
#password_query = SELECT password FROM mailbox WHERE modusname='%u'  AND active='1'

user_query = SELECT \
    '%u' AS master_user, \
    CONCAT(mailbox.storagebasedirectory, '/', mailbox.storagenode, '/', mailbox.maildir) AS home, \
    CONCAT('*:bytes=', mailbox.quota*1048576) AS quota_rule \
FROM mailbox,domain \
WHERE mailbox.username='%u' \
    OR mailbox.modusname='%u'\
#    AND mailbox.domain='%d' \
    AND mailbox.`enable%Ls%Lc`=1 \
    AND mailbox.domain=domain.domain \
    AND domain.backupmx=0 \
    AND domain.active=1 \
    AND mailbox.active=1

The other problem was the domain login mismatch. There's a feature in postfix which prevents you sending mail as a domain that you're not authenticated as. The quick fix is to turn it off, but that allows you to send mail as obama@whitehouse.gov if you feel like it. I don't think wefigured out a way around that before we dropped the whole plan

also, here's the modified query from /etc/postfix/mysql/sender_login_maps.cf

#query       = SELECT mailbox.username FROM mailbox WHERE mailbox.username='%s' OR  mailbox.modusname='%s' AND mailbox.enablesmtp=1 AND mailbox.active=1
#AND domain.backupmx=0 AND domain.active=1

but I don't remember that working particularly well