Emails on my smartphone ~ not 100% working?

[#1]

Hi all
I had one email account setup on my smartphone and needed to add another, the checking and validation worked fine for both POP3 and SMTP but I have since realised today that I can only receive emails not send any.
However the oddity is weird, as in when on WiFi all is well but when on 3G I can only receive!  As I missing something as I am fairly sure when I originally setup the first account last year it was 100% OK on 3G.
So why only "two way" on WiFi and no longer the same on 3G???
TIA for any insight and help with settings changes but as noted even on 3G the POP3 and the SMTP validate OK with the servers.

[#2]

Working on home WiFi but not on mobile 3G suggests you haven't set up authentication on the outgoing server. That's assuming your mobile ISP isn't blocking access to remote relays on Port 25 or Port 587, whichever you used with relay.plus.net.

[#3]

Thanks for the reply
Whilst waiting I went on a hunt for any possible solutions and found the PN page on the portal about setting up an Android phone.
It appears I needed to setup SMTP authentication sign in, that appears to have solved the problem on both accounts but of note I am sure that the account I setup last year was AOK without the sign in ticked on the SMTP, so did PN make any changes since say 1st Jan 16???

[#4]

I think I've been using my Plusnet email account on smartphones for a couple years now and I've always needed to use SMTP authentication to send mail when connected to my mobile phone operator's network. Of course it wouldn't be needed if connected wirelessly to my router as then I would be on Plusnet's network.

[#5]

SMTP authentication for sending emails over a non-Plusnet connection has been required for lots of years. Without it the server would effectively be an open relay with *all* messages sent through it likely to be treated as spam.