Either will work - (anything)@btinternet.com or (anything.else)@btbroadband.com for residential ADSL/VDSL. Business users have a specific username and password, as noted up-thread.
From September 2001 on BTopenworld Home 500/Home 1000/Home 2000. Then ADSLMax on <n>ildram. Moved to ADSL2+ from ADSL24. I'm now with plusnet. I'm not saying who I work for. Any opinions expressed here are my own.