How can I store email on my LAN and allow client-agnostic access?

Question

I have a larger than typical LAN with multiple versions of Windows, Ubuntu, and multiple types of devices (PC's, tablets/phones, appliances, etc.). I want to create a centralized mail "repository" to allow users to house their email in an "Exchange-like" fasion. I want to be careful here to specify that I'm not looking for an Exchange alternative, per se. I don't require any particular feature set aside from being able to fill a "centrally managed mailbox" role. I want to be able to:

• create mailbox accounts that, in some fashion, fetch messages from other accounts (ISP, gmail, etc.) to be housed in the mailbox
• access this mailbox with any client (or as many as possible; "any" can be a steep order) and not have the account's emails become "fragmented". (eg. a message that was read on a client is then only accessible from that client)
• sending email will be handled by ISP (they block port 25 on my connection) and is not a requirement

After some initial research, I think that perhaps an IMAP server might fit the bill (but I could be totally wrong about that). I understand that, after selecting a platform, there's a whole 'nother rabbit hole of learning to configure it, but this question is about the platform.

What are my platform options for creating such an email server?

Answer 1

IMAP is practically the only choice, yes. It keeps all message state on the server, allows organizing mail in folders, and practically all mail clients support it, often with real-time updates as well.

For devices that don't (e.g. if they only have a web browser), there also are a few webmail clients that use IMAP themselves, such as RoundCube, SquirrelMail, or Horde IMP.

I haven't tried many IMAP servers, but so far Dovecot has worked very well for me. IMAP servers usually cannot fetch mail from elsewhere, though; they expect it to be delivered by a MTA like Postfix or Exim or Sendmail (which receive mail by SMTP) or by programs like fetchmail or getmail (which download messages from another IMAP or POP3 server).