Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
AuthSMTP - AuthSMTP is the outgoing SMTP email service for e-commerce websites, mailing lists or email applications.
PowerMTA - PowerMTA is an industrial-strength software for high-volume email delivery.
BriteVerify - Email Validation & Email Verification reduces bounce rates up to 98%. Rapidly verifying email addresses has never been easier, just drag drop and deliver!
EmailSuccess - EmailSuccess is an advanced MTA built to handle the most critical reliability, performance, security, and deliverability requirements.
CritSend - CritSend is a SMTP relay for developers.