Not sure that’s the issue Cougar as it works on this home wiife, seems to be using BR SMTP on a none BT connection.
Probably because it’s BT email address on a BT connection, they already know who he is.
I suggest changing his SMTP details to the same as his icloud account.
Yeah, this is the other way of doing it.
To send email successfully you either need to use the service provider’s (not email provider’s) native SMTP server, or you need to successfully authenticate to the SMTP server before sending. Eg, back in the day, say you were using Yahoo email over a Pipex dial-up, you’d typically use Yahoo’s POP3 server (because that’s where your mail was stored) but Pipex’s SMTP server (because they know you’re a valid customer). So using your iAccount should work so long as a) you’re able to successfully authenticate with it and b) Apple aren’t doing anything fruity with mail relay.
To be honest, the real solution is to bin off the BT email address and use Gmail / Hotmail / Proton Mail / any one of a million other free email providers. Otherwise it ties you in to your ISP, so if he ever wants to change service providers he’ll have to change email addresses (or continue paying BT). And the longer he uses it, the worse it’s going to get when that happens.
Plus, y’know, webmail. I can’t remember the last time I used a full-fat email client for non-work email on a desktop device.