So out of the dozens of email accounts I setup on my new Debian server with postfix/dovecot one of them was unable to send emails.. No bloody idea why, maybe these guys were the only ones using outlook express, I don’t know but the bottom line is that none of the emails were going out as they were all coming back with the following error:

NOQUEUE: reject: RCPT from unknown Relay access denied; from

After tonnes and tonnes of trial and error I narrowed it down to /etc/dovecot/dovecot.conf , much thanks to this post : http://www.linuxmail.info/postfix-smtp-auth-dovecot-sasl/

In the end I needed

mechanisms = plain login

near the end of my .conf file so after the edit it looked like this:

auth default {
user = root

passdb sql {
args = /etc/dovecot/dovecot-sql.conf
}

userdb static {
args = uid=5000 gid=5000 home=/home/vmail/%d/%n allow_all_users=yes
}

socket listen {
master {
path = /var/run/dovecot/auth-master
mode = 0600
user = vmail
}

client {
path = /var/spool/postfix/private/auth
mode = 0660
user = postfix
group = postfix
}
}
mechanisms = plain login
}

Yay, hopefully this is the last of the problems with this mail server