SMIME: smime_vxst.cer (It's not as secure as GPG, but at least the email provider can't read your mail. It's the preferred encryption method since I can access it on the go, and it's safe enough for most use cases)
You can always send me an email at me@vx.st, signed with your SMIME certificate, to request my signed reply, which will establish a trust tunnel for the future. This process is very easy with iOS/Apple Mail.
Since my GPG secret key only exists within hardware tokens, I'll need a computer to decrypt the message. You'll wait more time before a reply (once a week), but this method is literally unbreakable.