Mail features that Gmail doesn't support (yet)

• Recipient-based mail formatting. Your boss wants you to attach an nice signature, but you hate HTML. With offline clients (Thunderbird, Outlook…), you can decide to reply with HTML for a given contact and with plain text for others
• Sender-based recipient address. You have configured two addresses for sending: me@home.org (default) and me@work.com. Way too often, you accidentally send a professional email with your personal address. The only known software I know which can prevent this is the incredibly-powerful-but-scaringly-CLI mutt (and the even more powerful mutt-ng)

Want them? Ask for them!
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Recipient-based mail formatting. See http://goo.gl/yyR6Z

Sender-based recipient address. See http://goo.gl/yyR6Z