@Lawrence Finch, you're not being helpful. Please advise folks coming to this thread with this problem to simply switch off "Delay Sending" in the Mail Settings. That fixes it.
You suggest, "...either no password or the wrong password in your outgoing mail settings ...Usually the easiest way to fix the problem is to delete the email account ... restart your phone...add the account back". I'm concerned for the poor folks doing this, and simply re-installing the default "Delay Sending" and thus not solving their problem.
Regarding "force quit" (on iPhone I guess you mean swipe-up a running app, whereas on OSX it's an actual command) it seems as if you assume that Apple does a poor job of cleaning up all the bits-and-pieces required to run an app. I actually assume the opposite: Apple does a good job of cleaning up loose ends when an app is closed, cancelled, or quit (forced or otherwise).
You may be drawing on Microsoft experience, where apps often leave processes running after an orderly or disorderly close, sometimes requiring a re-boot or even manual identification, detection, and killing of the process (and a gaping hole for hackers). This is extremely rare in Apple-World (and thus inherently much more secure).
Folks, if your Apple Mails on iPhone are not sending, go to Settings, Mail etc and switch off "Delay Sending". Simple. Done.