Outlook does this and it's a big help.

Selectable font and adjustable type and size on Mailbox like making a mail. Mailbox isn't clear now.

It would be great if emails were already synced when I open the app through background app refresh. But it'd be nice if this were a choice.

A font size selector (similar to tweetbot) would be great. I'm running Airmail and Outlook and prefer the smaller font in Outlook.

Prepare the mail And schedule the sending like in Outlook or like the plug in "boomerang" for gmail.

Handoff as with the apple apps.

Synchronizing with calendar.app (and then with calendar on Mac) to programming time to send determinate emails and connect one or some emails with one or more appointment.

It seems that airmail completely polls every mail account every time the app is started. After four app starts app. 100 MB of my data volume is used.
You should add an opttion to reduce the traffic or save more data localy and only download new or changed messages.

Fully use all the features that Gmail offers. To tag and move emails to their respective folder easily, and being able to send emails through different alias.

Outlook (iOS) has this feature and I think it's awesome. Airmail should be able to parse the mail and if there is an "Unsubscribe" link, there could be added a line w/ text or a button above the mail where I can unsubscribe easy and fast by tapping this link / button.

Integration with Pocket Informant and Omnifocus

I'd love to have the option to schedule my messages. Read receipts isn't a bad idea either. Some basic Yesware style functionaly would be welcome.

I would love to have an email client that tells me whether the recipient has opened the email. Gmail plug-ins like 'Streak' have this. Ideally the read receipt happens silently without the recipient having to approve another outgoing mail back to the sender.

Let the user configure more snooze options and chose a default one if short or long swipe

Sparrow was a bit of a resource hog, and could often slow down. Apple Mail is always snappy, and was the primary reason I switched back. Please keep Airmail iOS fast!!