Hopefully the link I posted will help with that, but I can't offer any direct help I'm afraid as it is beyond my expertise.
Thanks for your reply. However, I don't understand the distinction between "I am sending from my own address" and "using a BT or Gmail SMTP server". How do I go about doing the latter?
Unless you have your own domain (and hence proabably own mail server address) - it's what I do - then in order to send emails from your connection, you have to use BT's SMTP server. You have to effectively make your pi client have the correct configuration to use the proper servers along with the email address/password. (You can use Gmail stuff, possibly outlook.com and others).
Virtually all of BT's IP ranges will be on most blacklists, so if you don't use a proper mail server - virtually all of your emails will fail to be delivered as the destination will be using blacklists. Your IP address in the email header will show it being used as an SMTP server - and hence delivery will fail (it's a bit more complicated than that, but in simple terms ...)
These blacklists don't typically contain the IP addresses of ISP smarthost servers (e.g. mail.btinternet.com) - hence email can be delivered.
Thanks for the link which looks interesting. It will take me a couple of days to work through it after which I shall post back.
Well done, I did have a play but getting an error message when I test sending. It seems to point to Sendmail not reading the authinfo properly but I haven't had chance to investigate fully.
It did not work first time for me but then I read the blurb at the top of the submit.mc which led me to run sudo makeconfig after changes to the file. This seemed to work. Again, thanks for your help.