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Trying to estimate specs for a sendmail box

We're estimating an eventual 3000+ email boxes over several domains running on a rack server with a 100Mbps internet connection. Probably 30-50MB mailbox limit, and supporting secure connections (though will won't be used by clients often).

Any ideas on how to estimate processor, memory and hard disk space? We need to keep costs as low as possible.

We may purchase a dual CPU motherboard but only one CPU, then add another later if necessary - though it'll be hard to get exactly the same model etc.
1 Solution
Is the same server responsible for receiving E-Mail also responsible for local delivery (i.e. putting a received E-Mail into a mailbox)?

In a hosting situation like you describe, I'd go with separate servers for these function:

1) a mail relay (at the logical network border/in the DMZ), responsible for sending E-Mail out and receiving E-Mail in for the hosted Domains, but not storing any E-Mail for longer than it takes to relay to its next hop. This disk space requirements are relatively low, but CPU/memory could be high depending on anti-SPAM/AV defense requirements and expected mail volume.

2) a mail host, which actually holds the end-user mailboxes. Disk space requirements here are high - if you have a 50 MB limit and 3000 mailboxes, that's an easy 150 GB, and since most people regard E-Mail as a divine right, go ahead and get 300 GB :-) But CPU and memory requirements are more modest, driven mainly by what access methods you'll allow to the mailboxes (e.g. POP, IMAP, shell, whatever) and how many simultaneous users you expect.

Splitting the functions - mail relay and local delivery/end-user access - across two servers makes it easier, IMHO, to provide effective anti-SPAM and A/V services, and makes it harder to DoS your mailsystem. The biggest drain on your mailsystem resources will be scuzzball SPAMmers - by splitting things up, you don't force your users to compete with the SPAMmers for access to their mailbox. The SPAMmers will be hitting the relay, not the mailbox host. And the mailbox host should not be accessible via port 25 from the 'Net, only via the ports needed for end-user access to mailboxes (e.g. TCP/993 for IMAP-SSL.. you DO plan on using secure mailbox access, right?)

How beefy do these need to be? Good question.

For the relay, if you have 3,000 *active* mailboxes across a number of Domains, you can expect at least 8,000 legitimate E-Mails to be sent/received each day. On top of that, once the SPAMmers find you, you can expect as many as 40,000 SPAM attempts each day, probably more. The key to holding down your hardware requirements is stopping the SPAM as early as possible - at HELO or even sooner. The question you now need to ask is political rather than technical. What is the tolerance in the user population for SPAM, and for things like false-positives (where legitimate E-mail gets blocked). How much blocking are you allowed to do, and on what criteria (for example, can you use RBLs)? Or are you required to accept and delivery EVERY E-mail, even the obviously bogus SPAM? There's a big difference between having to process 8,000 E-Mails a day vs. 48,000 a day. Allow an open tap, and users will hit that 50 MB mailbox limit a lot quicker, too. So, you need to answer that question, and decide on what anti-SPAM/AV defenses you'll (be allowed to) employ, before you start specing hardware.

For the mailbox host, the disk space is easier to figure out than CPU/RAM. You need to look at the access methods and expected simultaneous clients. If you only allow, say, IMAP-SSL (CPU intensive) but expect a mere 300 clients accessing mail at the same time, then CPU is more important than RAM. If you expect all 3,000 users to be hitting it at the same time via plain ol' POP3, then RAM is more important than CPU. Again, you need to consider access methods, access control, and number of *simultaneous* users before you start specing hardware.

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