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Whenever we try and send a larger mailing run (like 1000+), our Kerio 6.7 mail server stops accepting IMAP requests from clients.

Normally this has been fixed by giving the machine a reboot, but would like to fix it properly. Checked the logs when it stops we get repeated error messages of "DNS socket: socket limit reached." After the reboot the server webmail/IMAP is noticeable slowed until the queue is cleared.

We are still using ISP smtp relay for email delivery which limits us to approx 500 emails/hour, but would this cause the server to fall over? Using ISP as our (UK ADSL) IP was being blocked by some providers as being in a home IP range, so lots of mail wasn't delivered.

Currently running Kerio 6.7 on Apple Mac Mini 2.0GHZ C2D running 10.5.6, but this problem persisted before using MacPro with 10.4.7 (and earlier). Using Cutedge DNS Enabler for Leopard for DNS too.

TIA for any pointers

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OS X can only handle 512 simultaneous IP sockets. I'm not sure if it can be changed with sysctl, but try

sysctl kern.ipc.maxsockets=1024

and see if it helps. The reason your IMAP connections are refused is because the socket list is exhausted, and there's nothing you can do about it if OS X doesn't let you change the resource ceiling. Rebooting the server will reset all sockets, thus giving you a temporary solution.

How many SMTP delivery threads have you configured? The default is 32, and it's probably tempting to increase the number of threads if you do mass mailling. The trade-off is of course that you spend sockets on SMTP delivery that you need for IMAP, POP3, SMTP reception and webmail.
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