Re: Pesky emails
Yes, but that is because it is an email client, not the SMTP connection. As far as I know, all general purpose email clients such as Thudermail, Outlook, etc, will let you send from any email address and set up mail to allow you to configure multiple email addresses on a single PC.
The problem is not the email client, but rather that the outgoing SMTP server you use may place restrictions on the email addresses it allows you to use. Your ISP doesn't want to be seen as an open relay and doesn't want their customers sending out mails with forged sender addresses, so they may put restrictions on what you can send through their connections. When using BT as my ISP, I had great trouble convincing them that I should be allowed to send email using my own email address on my own domain, and not the btinternet.com address they had assigned me as part of my broadband package, and I had to use a clunky authentication mechanism to show I was entitled to use my own domain.
I don't think this is unique to BT. I think ISPs in general expect that if you are savvy enough to have your own domain, you should be savvy enough to have your own SMTP service as well. Added to that, the auto-config options built into many email programs assume you are using the same domain name for both incoming and outgoing email connections, and don't offer any option to use your local ISP-supplied SMTP, even though it might have far-better performance, virus scanning, etc, than the token SMTP service supplied with cheap domain names.