Mailspy allows for quick and easy creation, sending and tracking of email campaigns

MailBuilder is a simple library for building RFC compliant MIME emails.

Send mails via EWS, useful for corporation environment like ours which has outlook via office365, but firewall block SMTP (don't ask me why, dunno).

A nice helper when setting the styling for an email.

Send mail through multiple vendors

Gem that gives you easy integration with polish mailing service FreshMail (freshmail.pl)

An easy-to-implement, reusable newsletter subscribe widget that connects with Mailchimp.

A simple ERB template mail builder. It creates an email with inline images. Very nice =)

A microframework for mail-centric applications

Lita handler for interacting with mailgun.com

This Rails gem allows you to send X509 signed and\or crypted mails.

Powerful mailing list manager.

email-in for static blogs like Middleman. Uses Mailman to poll an account.

Simple unread mail checker gem.

Learn with Mailbox

Ruby wrapper library for GNU Mailman's admin functions

Tool for sending fast mailings in one SMTP connection

A mail utility so dense, no message can escape.

Override ActionMailer recipients so all mails go to a given address

Allows you to manage your rackspace hosted mailboxes, eg. create a new mailbox.