I have a sendmail server. I was wondering if it's possible to allow both POP3 and IMAP access at the same time (so that some clients continue using POP3 while some use IMAP do access their mail)?

Yes, this is possible. You need to properly configure your mail delivery agent (e.g. Dovecot) to support it.Make sure your firewall has ports open for imap, imaps, pop3 and pop3s.

Sure, you can serve both POP and IMAP on the same server, even with the same server software (courier-imap, dovecot, etc). Clients shouldn't use both at the same time - they should pick one or the other.

I'm using sendmail. I will look into the configuration settings for it. Thanks for the tips.

Sendmail isn't your Mail Delivery Agent (MDA), and doesn't speak POP or IMAP - it speaks SMTP. Your MDA will be Dovecot, Courier, Cyrus-IMAP or something similar. Sendmail is the MTA (Mail Transport Agent) that servers use to talk to each other (via SMTP), and MUA's use to send messages. To retrieve/read/download/move messages you need a Mail Delivery Agent - e.g. Dovecot, Courier, Cyrus-IMAP et.al. - and these products speak POP3 and IMAP to the clients.

Hope that helps you

As far as I know, sendmail gets the incoming mail, then sends it through spam-assassin, which sends it to another instance of sendmail which I suppose might send it to the MDA you speak of. I thought sendmail was functioning as MDA in its second instance, but perhaps not (if not, why wouldn't spam assassin send the mail to the MDA directly?) My old boss outlined the situation before I left, but I haven't had a chance to spend much time with this server yet as I've been busy with other things.

In any case, sendmail isn't going to act as an imap or pop server. Yes, MTA's can deliver directly or go via a number of mechanisms to deliver mail (eg maildrop, procmail).

This doesn't change your need for a suitable package for serving imap and pop, so a check as to whether you have any of the packages blacky suggests would be a good starting point as they are likely candidates (add uw-imap to that list)

Sendmail is responsible for sending/receiving emails, but users connect to your server via something else to read their emails. I'm a Postfix guy, so I'm not a sendmail expert at all, but Postfix and Sendmail are both MTAs - not MDAs or MUAs. Look here for a basic block diagram of how email works. Sendmail does the server to server stuff using SMTP.

Sendmail will also do all of the SpamAssassin work to cleanse the emails before they are passed to the MDA for delivery to the MUA (whenever it might connect). It can also use RBL and DNSBL to block spam at your front door.

You need to dig a little deeper on your server to see which MDA you are using. Use the command "chkconfig --list" to see what packages your server is running at your current runlevel (usually 3 for text only, or 5 for GUI). You can also use "nmap localhost" to see the open ports on your server now (look for 110 and 995 for pop3 and pop3s, and 143 and 993 for imap and imaps - 25 and 465 are smtp and smtps, but smtps is not typically implemented). Also, use "netstat -lnp" to see the programs that have those ports open. You should see that sendmail has port 25 open, but something else will have 110 or 143 open.

Examples of MDAs include Dovecot, Courier, Cyrus-IMAP and others. Examples of MUAs include Outlook, Thunderbird, Evolution and others. MUAs talk using POP3 or IMAP to the MDA to read/receive mail, and talk using SMTP or SMTPS to the MTA when sending mail.

Let us know how you get on. Also, what distro/version are you using?

The distinction of MTA/MDA is difficult for new users, especially when a software package provides functionality of both. Eg. Postfix is both an MTA and an MDA (local, virtual, pipe agents are all MDAs). Further confusion abounds with multi-functinonal packages such as Courier, which is an MTA, MDA, and also provides POP and IMAP!

Agreed, email is easy to use, but a complex beast to understand, implement and manage. Hopefully the chkconfig and netstat output will help us find which MDA is actually being using.

I will get back to you guys on this (thanks!!) asap, but I have a number of more immediate concerns to address.