Gmail POP and SMTP server settings.
Mandrill, deliver incoming email to a webhook, and process outgoing email. Can do message manipulation also. 12k emails/month free, from MailChimp.
Hosted WordPress service which includes an affiliate program that pays 20% for the lifetime of the account, plans start from $10/month.
Controlled Mail is an outbound mail service (SMTP provider) with a very strong focus on genuine mail delivery. They accept no bulk mailings whatsoever and support SPF and DKIM by default. They also prevent cross client forgery.
An open source mailer class for PHP, supports local sendmail, smtp, the full monty.