Mac Mail - just stopped sending email

[#1]

I use a MacBook Pro and just this morning, without having changed any settings whatsoever, it stopped allowing me to send mail out through my PlusNet account. Admittedly, I am not on my PlusNet broadband, as away from home, but this normally makes no difference any other day of the week when at work. I have no problem downloading (receiving) emails from PlusNet. 

Does anyone else use Mac Mail and experience regular problems like this? It happened a few weeks ago as well. I am just surprised that nobody else seems to have reported it today. Thanks for any help. 

[#2]

I would add that, now that I'm home, there is no problem sending email with the server settings remaining the same. 

[#3]

Admittedly, I am not on my PlusNet broadband, as away from home, but this normally makes no difference any other day of the week when at work

are you on a mobile broadband or public WiFi connection ? It's possible that the public ip you are assigned has a bad reputation from previous users. In this situation, the pn outgoing servers will reject mail submission for a period , usually around 24 hrs

[#4]

Is it the same network you usually use when away from home that it works on (ie your office), or do you usually use it on a number of different networks away from home?

If it's usually just the office network, then do you know if they used to use Plusnet and if so do they still use them?

Reason for me asking this is, to send emails with through the Plusnet servers you need to either be on their network or use your username and password to send.  One thought I've had on this is if you haven't specified your username and password, and your office had a Plusnet connection, then that would have all worked.  If they have now changed ISPs (possibly because Plusnet Business is closing), then you wouldn't be able to send with them.  If so it's a fairly simple fix of entering your username and password in your settings against the outgoing mail server.