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A fast, highly extensible, and event driven SMTP server
Event-driven networking engine written in Python.
Simple, modern & fast web-based email client
A cross-platform .NET library for IMAP, POP3, and SMTP.
yagmail makes sending emails very easy by doing all the magic for you
Mini SMTP server written in golang
Teaching an Old Dog New Tricks -- IRC: #neomutt on irc.freenode.net
Papercut Simple Desktop SMTP Server
Inbucket: disposable webmail server (similar to Mailinator) with built in SMTP, POP3, REST servers
VMime Mail Library
qpsmtpd is a flexible smtpd daemon written in Perl