A popular front-end mailer, a program that accepts calls via modem and either transfers echomail packets or redirects calls to a BBS program, as appropriate. Frontdoor and the closely related Intermail, which was written by some of the same programmers, were probably the most popular choice of front-end mailer. They weren't extremely easy to set up - lots of obscure and oddly documented text files for some things, an interactive configuration program for others, cryptic error messages that make troubleshooting difficult - but these were problems endemic to almost all front-end packages, and Frontdoor was so widely used that help was easily available from other sysops.