I haven't seen this feature in any client (web or desktop), only services such as Boomerang, which are costly and cumbersome. The idea is pretty simple: add a search criteria for "unanswered emails."
This could later evolve to marking emails for follow-up (and use this mark as an extra filter to the proposed feature).

Please add drag and drop attachment from email on desktop. It is not possible to drag it, only with the right click save to destination.

Helps with fixed-quota MS Exchange accounts. Allows storing offline emails in a MS-supported way.

The "Today" mailbox is useful (to me at least) but I wish there was a "yesterday" inbox and probably a "last 7 days" mailbox too, to help me implement a GTD-inspired daily and weekly routine.

This would be a view that previews all attachments for an account.

I personally find hunting down attachments to be one of the most common (and most annoying) tasks when looking through my email.

A dedicated attachments view would solve this problem.

The signature designer is pretty basic...importing photos is particularly problematic. I'd like to see a more robust signature designer,

Markdown e-mail is an excellent feature. For us scientists out there, supporting MathJax in e-mail would be an amazing addition. A connection to the public MathJax secure server would be sufficient.

Thanks for giving this some consideration!

Please include the possibility to customize the Reply Header of an email, a template with a set of variables to choose from.

Not all of us can see that well... Option to have larger fon in folders and elsewhere

Save the email attachments and Airmail 2.0 opens the first (or only) attachment. eg A zip gets un-zipped, a PDF gets opened, etc.

I am choosing to save not open. This current functionality is not productive and is counter-intuitive - more-so because there is no user feedback that this is taking place (a large zip file can unzip and tie-up the Airmail app without any feedback).

If a folder has unread mail it be nice if Airmail would highlight or bold that label so that it stands out more similar to the way Apple Mail and Gmail works. Labels are a useful way to prioritize incoming email and since I've switched to AirMail I find it is easy to overlook new mail in certain categories.

I have lots of personal email addresses which I would like to see in one unified account but I would I like this kept separate from my work email accounts. Right now, I can either have them all unified or all separate - a third option would be great.

Actually, Airmail doesn't notify me when I receive mail that are filtered with "skip inbox" option in gmail.

This is trivial, when I reply to an email with a quote, I'd like to start typing below the quote.

Bottomquoting annoys some people on established mailing lists.

You already hide the default [Gmail] Root Folder, which is great. However, some german GMail Accounts have a different Root Folder named [Google Mail], which is not hidden but never in use.
It would also be nice if the root folder of Mailbox-App [Mailbox] could be hidden.

After I send a message, once in a while I realize I sent it to the wrong person and I would like to be able to unsend the message, if that is possible.

It would be great, if you could choose where Airmail should look for icons.
e.g. preinstalled Contacts.app