There is no reseller/subsciption/domain - specific mail - log, there is only ONE server - wide "mail.log" at "/var/log/", where the name and location is for example defined at configuration files when you use "rsyslog".
Yea... my Fault
We have Servers with only one Client on it.
The Customer has User SSH access....
Now i use a Cron to Copy the Maillogs to domain.de/logs/mail