Offline mail readers are computer programs that allow users to read electronic mail or other messages (for example, those on bulletin board systems) with a minimum of connection time to the server storing the messages.
BBS servers accomplished this by packaging up multiple messages into a compressed file, e.g., a QWK packet, for the user to download using, e.g., Xmodem, Ymodem, Zmodem, and then disconnect.
Off-line mail readers are generally considered to be those systems that did not originally offer such functionality, notably on bulletin board systems where toll charges and tying up telephone lines were a major concern.
Users of large networks such as FidoNet regularly used offline mail readers, and it was also used for UseNet messages on the internet, which is also an on-line system.
Less well-known examples include Silver Xpress's OPX, XRS, OMEM, SOUP and ZipMail.