The email BT send when my bill is ready partially obscures my BT ID with asterisks. As BT state in my BT.com profile "Your BT ID is also the contact email we will use to tell you your bill is ready". So they are sending an email to me at the email address I use as my BT ID and my BT ID appears in plain text in the email header. Why does it make sense to partially obscure the BT ID in the message body?
How does that answer the question, BTID can be any email address not just @btinternet.
@david644803 Your BTID isn't necessarily the email address you have elected to be contacted on. In your case it is but it is probably simpler to just mung every BTID rather than check.
The actual email address is irrelevant, in the OPs case the mail is being sent to an email address that is also his BTID regardless of whether that is @btinternet, Gmail etc. Agree it seems pointless to mung it, but this is BT we are talking about.