Can't send email from different connection.

[#1]

I'm away at the moment and both myself and other half have our laptops with us. I can't send emails OH can. I thought it was probably a BT thing [ISP we are using] but the difference between our accounts is the OH downloads his emails from MAAF and mine are forwarded to PN and downloaded from there, we can both receive emails.
Is this a PN restriction? I can't find any difference between any other settings.

[#2]

Which ISP is the subscription actually paid to? On Plusnet SMTP authentication works from a non-Plusnet connection only when subscription account credentials are used for sending emails (via relay.plus.net). Sorry, I don't know how MAAF relays handle this.

[#3]

No idea it was holiday accommodation but the siid BT so assume it was them.
Am using relay.plus.net as per PN set up instructions

[#4]

Is the PlusNet account which is the subject of this discussion a paid for account?  If it is not, you have no chance of using the relay.plus.net server if not connected to PlusNet directly.
If it is a paid for account, then the SMTP (out bound) server details are wrong.  (1) You ought to be using relay.plus.net port 587 as the server and (2) be using authenticated connection, using the PlusNet account name and password.