If you have your preferences set to be notified when someone replies to your posts then you will receive an email to that effect. You can now simply reply to that email and your message will be posted to the forum.
There is a time lag between message cycles of 15 minutes imposed by the forum software. Ten minutes are allowed for editing posts if you are using the web interface and this applies to email responses as well. Then another five minutes is built into the timing of email polling.
If you are posting using email providers like gmail and outlook then those impose additional, unpredictable time lags as well.
The bottom line is that posts via email may take a few minutes to show up as replies in your inbox.