Hi.
Problem such.
The microtic is connected to the Internet through vpn, but at such connection it limits connection on smtp, pop3, ftp. But if to be connected to it from a local network too on vpn all it is normal.
Possibly an MTU problem? 1500 byte packets will not fit down the VPN tunnel without breaking them up, and if the sender uses DONT FRAGMENT and blocks icmp than you are in trouble.