How to send mail from a non-BT address from a BT account
Problem: You have a BT dialup or broadband account, and you usually use the email address you got with this account for your mail (such as email@example.com). You want to use a different email address (such as firstname.lastname@example.org). When you send a mail from the different email address, it bounces back and is not delivered. If you examine the bounce message, you see something like the following:
----- Transcript of session follows ----- 553 5.1.3 email@example.com... Invalid route address
Solution: call the BT helpdesk and request that they permit you to send mail from your other address. You need to do this for each different address you plan on using.
Alternative solution: Use a different ISP - it's only BT that impose this restriction.
Background: BT don't let you use any other address to send mail, so when you use your other address, they block you and give you the 553 error.