Iphone/plusnet email - cannot send

[#1]

The following issue seems to have flummoxed PN support (ticket 84700994)
For several weeks (after 18month of faultless service), I cannot send PN emails from my iphone5.
1) I have a iphone5, operating on o2. I have checked my set up with O2 and BT (phone provider) - all is well. I have a works email, and a gmail account - both operate correctly.
2) I can recieve emails on iphone, i cannot send - "outgoing server rejects recipient address"
3) my plusnet account works well on my home PC
4) Until 2 or 3 weeks ago all worked beautifully.
5) i have repeatedly deleted and set up plusnet email account. I have deleted any spurious outgoing servers. I have changed all passwords. I have tried with outgoing server user name and password left as "optional" and with these entries complete.
I am not rather stuck! Any help is very welcome!
Googling suggests an iphone issue - and a lot of people seem to have been searching for a solution without much joy. Certainly no consistent issue has been identified.

[#2]

Hi canary2211,
Can you just confirm a couple of details with you...
Are you trying to use the PlusNet SMTP server (relay.plus.net) from your phone?
Does this work when connected to your home network via WiFi?  If not you need to get this working first.  Usename and password should be the same as that for the incoming mail server for that mailbox.
If it does work, have you configured it to use port 587 (in place of 25) with username & password configured and non-SSL?  If not please do so.
When you say it does not work, is that in a location using 3G / 4G connectivity?

iDevices are known for messing up their email configurations.  No matter how many times you input and believe to have correctly set up the details it does not work.  The only resolution is to drop the email box entry and start again.  It is always best to attempt such a set up when connected to your "base" PN account network.

[#3]

Hi Townman
Thank you for the help!
I have got it working perfectly well at home wifi. 
Yes it is relay.plus.net
Yes port 587, with username and password and non-SSL.
I turned off wifi - and tried it on GPRS (very old school) - and it worked.  We shall see how it goes tomorrow in various locations with 3G, 4G and wifi.  I shall report back
Nick

[#4]

Townman
this all seems to be working OK!  Thank you!  Couldn't have done it without your help!