More Infos here:
https://www.redteam-pentesting.de/de/advisories/rt-sa-2013-001/-exim-with-dovecot-typical-misconfiguration-leads-to-remote-command-execution
but it's a dovecot problem as it seems. On the other hand "`" is not an
allowed char in an emailaddress..
How about just kicking those connection attempts by default with i.e.
"500 illegal chars used" ?
Marius