Your box is full.......

[vintage hunter]

Have you checked that there is still space in your "out-mail box". This is because you have an "in mail box" and also an "out mail box". If not you may need to delete some messages that you have sent in the past.

Yes, I've cleared both in box and out box of all old messages. I'm working from my phone in the computer version mode. That may be the trouble.