email server load

I don't think that the number of mail accounts is the key factor. Most important is the way mail is handled after receipt – spam and virus scanning are much more intensive activities (with respect to CPU and memory) than the actual delivery of messages.

The reason I ask is that I have a client that would like me to build them a dedicated email server as an off-site managed service. I'd like to go with linode, I just need to make sure it'll be able to handle 100, 200, 300 accounts.

I think that a few hundred email accounts will be no problem as far as CPU and memory go, but you may find the disk space on linode to be a problem. With free email accounts offering multiple gigs of storage, many users expect to be able to leave lots of mail on the server indefinitely. I would consider the amount of storage you expect to allocate per user and see if that will fit within a linode that is within your budget.

The limiting factor is the rate of email deliveries and the mail processing that you do on the Linode. Things like SpamAssassin and clamav use a lot of memory and CPU. If you are going to use them on a system that processes al lot of mail, you need a big Linode.

@pclissold:

Things like SpamAssassin and clamav use a lot of memory and CPU.
I'll second that. I'm running an IMAP proxy for my @linode.com mail, and that entails a getmail process which forks a spamd check per message. Needless to say, @linode.com gets a lot of mail – I noticed the performance difference at once.

Some non-resource-intensive ways to filter e-mail:
* Sanity checks

• Greylisting

• DNS block lists
  Stever has a good point about storage space. If you run an IMAP server with users who don't delete their old mail, it's probably not going to take too long to start filling up.