Hi!
I'm working on simple mail system which will support over 50 accounts in one domain.
Main assumptions of the system are:
-Supporting 50 accounts
-Deleting older than N days messages
-Retrieve messages using POP3 protocol
-Synchronize all users using IMAP

Which mail retrieval agent mpop,getmail or fetchmail will be the best solution, maybe something else ?

Do you have same idea how to make that system efficient?

When storing, accessing or retrieving mail the amount of accounts doesn't really matter. Deleting messages that are N days "old" only applies to POP as with IMAP you tend to keep messages as long as possible. (Unless you forgot to explain something?) And "synchronizing mail accounts with IMAP" doesn't really apply either as you would usually just provide IMAP as an interface, then it's up to the client to do whatever they want with the IMAP store. That said both getmail and fetchmail are protocol-agnostic, meaning they both deal with POP3(S) and IMAP(S) easily. Personally I find fetchmail easier for bulk fetch operations across multiple accounts and providers and keep getmail for syncing individual accounts like between a Gmail account and a local IMAP store... (Without explaining what "best" means to you I guess that's more like a "whatever works for you" kind of answer ;-p)