BT Internet's mail servers only allow messages to be sent from 'approved' email addresses. If you try to send messages from other addresses BT's mail servers may return a '550 Not Permitted' error, a '553' error, or may silently discard the message depending on what their server feels like doing today.
This can happen if you are forwarding messages out to another email address. In this case, see this article.
Or, it may happen because you are using an email address which BT don't recognise. In this case, see this article
We have no confirmed information that this helps, but it may be worth turning off BATV support in VPOP3 (version 5 or later). BATV changes the local part of the sender's email address so that legitimate bounce messages can be verified. This is a very useful feature for preventing spam “back-scatter”, and does not cause problems with sensible mail server configurations, however it may cause a problem in this case. To turn it off, go to Settings → Misc Settings → BATV, and uncheck the 'Enable BATV Support' box. Note that BATV has to be a global setting because of the way it works, so if you send mail through several different mail servers, you cannot disable it just for one.
If none of these help, you may need to contact BT technical support to find out exactly what their error message means in your particular instance. Alternatively you could use a different SMTP relay service. There are several around, such as http://www.authsmtp.com, or we can provide an SMTP relay service for VPOP3 users starting from £40 a year (contact support@pscs.co.uk for more information).
The above information has been useful in the past, but is not guaranteed to help, since BT may change their server configurations at any time. Since we have no access to BT's mail server configuration, we cannot know what rules their server is applying at a particular time.
Note that as an alternative to mail forwarding, a better way may be to allow remote access directly into the VPOP3 mailbox. See
If you think VPOP3 should be sending mail from a particular email address, and want to confirm this, you should go to Settings → Diagnostics → Session Logs, and turn on 'Log SMTP Client Connections'.
Now, VPOP3 will record its conversation with the ISP SMTP server to a log file called SMTPCLT.LOG, usually in the VPOP3\_LOGS or the VPOP3 installation directory. You can open that log file in a text editor, such as Notepad. If you look for a line containing 'MAIL FROM: ' that will contain the sender's email address which VPOP3 is sending to BT's mail server, which will let you confirm that it is using the correct address.