When sending a reply that is digitally signed, the message may either 1) get stuck in the Outbox, where you can try to resend it, or 2) result in the message, 'Cannot Send Mail. In iOS 13.2, when replying to a message with a digitally signed reply, the message may not send. This problem does not affect Mail itself, so you can use it to send signed messages, including attachments, normally. This will be the case even if you have configured the Mail app to sign messages by default. In iOS, if you use the share function (usually represented by a square with an upward facing arrow going out of it) in apps such as Photos to send mail, the messages will not be digitally signed. In iOS, mail sent from apps other than Mail is not digitally signed.
