I noticed that GMail is warning about missing attachments and heard that Thunderbird does this, too, from Arber Borix, who also responded to a request for a screenshot (below). Thanks, Arber!
Thunderbird: Found an attachment keyword: attached. Add Attachment... Remind Me Later
May 7th, 2012 at 8:40 pm
Thunderbird is, in fact, getting better. Its add-ons could make it comparable to GMail in terms of organizational power, I’d suppose.
May 7th, 2012 at 11:18 pm
@jschneider Introduced in Gmail Labs in 2008: http://gmailblog.blogspot.com/2008/09/new-in-labs-handy-intern-tweaks.html Graduated in 2010: http://gmailblog.blogspot.com/2010/02/gmail-labs-graduation-and-retirement.html