if (open (LIMIT, “/etc/virtual/limit_$name”))

If you have that line in your exim.pl, then you’re good to go. If not, save it to /etc/exim.pl and restart exim. The same basic rules apply, you just need to type:
echo 300 > /etc/virtual/limit_username

where username is the User you wish to give the special limit to. This overrides the default /etc/virtual/limit file for that User.

If you think you might have a spammer, check your /etc/virtual/usage directory for a larger than normal filesize.
Any authenticated sends will show up with this command:
cd /var/log/exim
grep ‘A=login:’ mainlog* | less

which will show all emails sent from your server which used smtp-authentication. This is useful to find any email accounts who’s passwords may have been compromised (guessed), or simply just abusive Users.

With DA 1.42.0 and /etc/exim.pl version 13, you can use a per-email send limit, where you can limit each individual email account to a daily limit, eg:
echo 50 > /etc/virtual/user_limit
echo user_can_set_email_limit=1 >> /usr/local/directadmin/conf/directadmin.conf
/etc/init.d/directadmin restart