Rejected by server email error

[#1]

Im getting a 'cannot send, the recipient was rejected by the server' error message every time I try to send emails from my iPhone but only when sending on cellular, it's fine on wifi, is this a Plusnet problem or my network provider (Three)? 

[#2]

I think you've answered your own question - if you can send by wifi (provided by PN), but not cellular (provided by 3), it kind of points to a 3 problem.

[#3 · Accepted solution]

Contacted Three, it turned out to be my Plusnet outgoing server settings not containing my username and password. It says 'optional' in these fields and as far as I know I haven't ever had them populated before. No idea why it suddenly required this information but thankfully the issue is now resolved. 

[#4]

They are always required if you aren't connected to the Plusnet network.

Thanks

Phil