The software was originally released in 1996 as Calypso by Micro Computer Systems (MCS).
[1] Courier supported the POP3, IMAP and SMTP protocols, provided several features for HTML security and allowed to use multiple e-mail accounts from one mailbox.
[3] The company abandoned further development of Calypso, which was acquired in 2003 by RoseCitySoftware.
[5] An upgrade path was negotiated with PocoSystems to add Courier-type features to Pocomail and direct Courier users to migrate to PocoMail 4.8 as a replacement for Courier 3.5.
[6] Free versions of the unfinished beta, requiring no registration, were available at the Yahoo Courier user group.