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

If none of these help, you may need to use a different SMTP relay service. There are several around, such as http://www.authsmtp.com, or we can provide an SMTP relay service starting from £40 a year (contact support@pscs.co.uk for more information).

Note that as an alternative to mail forwarding, a better way may be to allow remote access directly into the VPOP3 mailbox. See

1. Allow remote access to VPOP3 mailboxes, or
2. Using a mobile phone with VPOP3