i.Scribe is an email client with an integrated contact database and calendar, supporting SMTP, POP3 and IMAP (including uploads). Security features include Bayesian spam filter, HTML controls and attachment protection. Having OAuth support (albeit unverified by Google), it can be used with accounts that require it instead of a password.
Cross-platform with clients available for Mac and Linux. Note that the free version is limited to one email account and 5 user-defined filters.
