Concurrent IMAP connections from MacOS

I note that modern email clients maintain multiple concurrent IMAP 
connections to the server, e.g. Thunderbird has a (configurable) default 
of (?) 5, which would speed up polling multiple inboxes on one server and 
allow faster message copying between multiple folders.

Excessive connections from one client, on the other hand, may be a 
dictionary attack.

I had set a per_source limit of 10 on POP and 20 on IMAP in Linux 
xinetd.d/*. That works fine with Thunderbird and Squirrelmail, Alpine 
etc. (we don't have big NAT'ted client farms). However, we've been 
getting connection problems from MAC machines and logging xinetd 
per_source_limit failures.

Is this a bug in Mac Mail ? Does it really need that many connections, 
and if so how many ? I'm reluctant to dedicate so much server resources 
to one or two users for no good reason, or weaken filters against network 
attack.