problem sending emails via 3g only.

[#1]

Ah! That explains it, authenticated SMTP to the plusnet servers is ONLY available from a subscription account login or mailbox. That's why the dialup account didn't work.
Glad we got you sorted eventually.
BTW you can use your BB login to send for ANY email account so you can still use the dialup email address you just need to use your BB account details for the SMTP authentication. In fact as I said earlier, create a separate mailbox on your bb account and just use that mailbox & password for smtp authentication on all email accounts.

[#2]

As MisterW said Plusnet allow authentication only on subscription accounts (whether 3G connected or using external WiFi). However since you can enter any From (Identity) address you want you can supply the dial-up address while using your subscription account credentials for authentication.
The username/password used for receiving emails does not have to be the same username/password supplied for authentication.

[#3]

I've setup a mailbox as you suggested and will check it once it's live.
Many thanks for staying with this topic. Your information was very welcome and I've now made sense of how the system hangs together.
Best wishes for a Happy Christmas & Happy New Year.
Alan
p.s.
How does this topic get closed?

[#4]

Topic's don't get closed as such, unless moderators lock them for some reason.

[#5]

Understood.
Sorry to throw another query at you but here's another oddity.
I had years ago set up extra email addresses under "my christian name", "my wife's christian name" and added the "mobile" one yesterday all under our Account name.

Subsequently my Samsung mobile can now send via 3G using Accountname+mobile as the o/g server login name but my wife's HTC can't. It can however send using Accountname+HerName as the o/g server login name. The appropriate different passwords were used.
Any ideas as to why her mobile can't send using exactly the same o/g server settings as mine?

[#6]

I can't think of any reason under Plusnet's control that would cause Accountname+mobile (with its password) to fail on the HTC. Sorry to ask the obvious but when the outgoing setting was changed SSL/TLS security didn't default to 'on' again did it?

[#7]

I'm afraid not but I'll try switching the Security setting on & off just in case. Is there any particular reason PlusNet don't use any Security settings since most other email providers do?

Just in case you're interested........
I must say that having helped other oldies with their Mobiles and Tablets and now setting up our new Mobiles I'm not too impressed with all the different ways the Incoming & Outgoing email settings are presented and the variation in the options provided. I presume the manufacturers are having a hard time keeping up with new hardware, software, standards and expectations. The other oddity I've noticed is that when the settings are auto-checked and appear to be acceptable (i.e. no error report) it still doesn't always work when trying to send a test email.
I used to create simple VB software for my old company before I retired and I learnt very early on to ensure that any unexpected errors need to give some helpful &/or meaningful feedback to the user. Not as easy as it sounds of course.