This may be related to a problem I ran into trying to get my iPad (and later an iPhone) to check mail on my personal domain. Rather than bore you with all the hair-pulling and mysterious symptoms I ran into, I'll cut to the chase.
What I learned was that my domain's mail server would not allow simultaneous SSL connections from the same IP address to the same port number for IMAP. All the devices behind my NAT router appear, to the server, to be coming from the same IP address.
Some Web hosts also limit the number of processes which can be spawned on a server, with each IMAP session resulting in one process.
My hosting provider, for example, limits me to 25 processes. In Leopard and before, each time I ran Mail.app to fetch my mail, one IMAP session was spawned. In Snow Leopard and later, Mail.app creates *two* IMAP connections to each server, spawning two processes per mail session. (Don't ask me why.) Using POP instead of IMAP reduces the server connections and hence the number of running processes.
