Issue with sending email via phone: please help

Is anyone aware of an email issue in which you can send, reply to and forward a message through POP mail only if connected to WiFi?

I have a couple of POP accounts (outside of gmail) set up in the email application. The last couple of days (with the WiFi off), I have tried to respond to an email through my phone, as well as forward another and it comes up with an alert that it is