I wish to find an MRA which will only download mail it hasn't previously downloaded (regardless of IMAP 'seen' flag) and pass them to a Mail Delivery Agent (MDA).
I wish to use a remote IMAP server as a constant e-mail storage which can be accessed by multiple MRAs and MUAs. Thus, if one client marks an email as read (IMAP 'seen' flag) it can still be delivered elsewhere.
Most solutions I have looked into (fetchmail, getmail, etc) allow the options of 'fetch all mail' (not ideal, for obvious reasons) and 'fetch unread mail' (some mails will be lost if read by another MUA). Neither option fits the bill for my use case.
Note: GNU Linux