Previously working email account now won't send mail

Question

I have several email accounts on both my iPad and iPhone. One of them on the iPad has stopped sending mail from one fo the accounts, but continues to receive mail. I haven't changed anything - and before I began to fault find, the settings on both devices were the same (with the spole exception of the Outgoing Server Port which was 110 on the iPad and 995 on the iPhone.
I have deleted the account on the iPad and recreated it - I changed the Server Port of 110 to both 995 and later to 25 (having seen on a forum somewhere else, being told that 'this will solve the problem').
Nothing changed. Can still receive mail into the account, but replies to messages and new mail messages don't leave - the error message is that the mail has been 'rejected by server.
On my iPhone, laptop the account works fine.
Anyone able to help - I've Googled but dont' seem to find any reason why a perfectly good, working account should suddenly stop working when nothing has been changed to cause it to stop, or any other advice other than changing the Outgoing Server Port. - Which I've done to both 995 and 25 (from the original 110)
TIA
Greg

Answer 1

John Burnes,
Port 25 may not work on some WiFi hotspots or the EDGE network, or may only send to some recipients but not others. Many email providers will also set up their SMTP servers to listen for authenticated connections on either port 465 or 587.
For example, .Mac does this because many consumer internet providers filter or block port 25 traffic:
http://docs.info.apple.com/article.html?artnum=305867
This article contains more information specific to the iPhone:
http://docs.info.apple.com/article.html?artnum=305634
This article discusses which ports some internet service providers use in the U.S.:
http://docs.info.apple.com/article.html?artnum=306074
For Germany:
http://docs.info.apple.com/article.html?artnum=306987
O2 offers their only relay service that you setup with them.
Hope this helps,
Nathan C.