Automatically choose account to send from
Apple's mail app chooses which account to send from in a smart way:
if I write a message to someone in the same domain as one of my accounts, choose this account
if I write a message to someone I have e-mailed with before, choose the account I have used before
if I write a message to someone who is in an address book connected to one account, choose that account
It's super annoying when I send a message to a work contact from my private email account and vice versa. With apple's mail app this almost never happens anymore. I don't think I can move to airmail without this feature

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Anonymous commented
Totally agree.
I must verify the sender address each time because Airmail NEVER picks the right one. How many times I've sent messages with my ultra private address to highly untrustworthy persons/companies, while I have hundreds of dedicated aliases (not kidding, not exaggerating)...
The basic rule would be "if I've sent a message with a certain address and the person responded, Airmail must keep the same address I previously used when I'm answering".
Airmail doesn't even respect that simple rule and picks whatever address it likes :|