I'm running a 1gb linode with cpanel installed. only a few lightly visited websites and a few email accounts set up for the moment.
I needed to upload 20000 emails from my computer to an IMAP account on the server, so I simply dragged them over from a folder in apple mail to the imap inbox. I believe this is an accepted way of doing this.

However while doing this all other email servises on the server stops responding. I can't send or receive email from any account on the server, even when trying to login from different IP address.
I can't log in to webmail or cpanel (time outs).
Apache websites works fine, and i can ssh in. load is something like 0.18 and cpu usage is low, is see imap-login at 2% CPU and 750mb memory free.

I asked buycpanel support, and they tell me it's too heavy work for the server to upload 20000 messages. And it is normaly that one user/account could block the whole servers email functionality. They even say "Even simply accessing a huge mailbox via webmail will be slow and it will overload the server a bit."

Any other opinion on what's the cause of this?

nick,
I think trying to upload 20k emails all at once would be too much. You could do smaller segments and see how well it works. Try 100 first and then bump it up a little from there.

My guess is that your email queue was packed, and with FIFO processing any other email would be stuck in line waiting its turn.

Did some searching and it looks like you should be able to export from apple mail to an mbox file, and directly import that into cpanel, skipping the imap server entirely.

Thanks, however I'm not so much after a solution for those 20000 emails, i have already uploaded them, just the server was unusable for other email accounts during the 6-8 hours the upload took.
I'm curious how this can completely shut down imap services for the whole server. What if one user on a big shared server does this ?

If a user does this it can 'clog' the iMap port until it's finished processing the incoming email synchronization. The port handles things 'first in, first out', so anything that is in line will get processed in order. This shouldn't affect your server's incoming mail (port 25) but it would impact those who use iMap to get and send their email. Any POP accounts and your webmail interface should have worked as normal, and any outgoing email sent from webmail or POP accounts should have worked normally too.

Uploading 20k emails all at once isn't a normal occurrence. There's probably a way to configure your iMap service (Dovecot, Exim, Courier, etc) to handle larger amounts of mail. Also, it's not just the number of emails but the size of them. If any of your 20k emails had attachments they went along for the ride.