Apache Commons Net library contains a collection of network utilities and protocol implementations. Supported protocols include: Echo, Finger, FTP, NNTP, NTP, POP3(S), SMTP(S), Telnet, Whois
Apache Commons Email aims to provide an API for sending email. It is built on top of the JavaMail API, which it aims to simplify.
Plexus Javamail Mail Sender
Plexus Mail Sender API
Socialmetrix Wrapper for Amazon SES SDK
Scala mailer built on top of the JavaMail API.