If you are a BT customer and had been using any @btclick.com or @btconnect.com mailbox before the migration to microsoft as the provider, you would (for example) have been able to send and receive email from your personal or business domain and manage an alias.
By logging into https://myoffice.bt.com/ you can change the name or title as it appears when logged in to the same account on https://portal.office.com. However you can no longer set an alias, or ask BT DNS to accept relay on your domain mail.
When logged into your .btconnect.com account/mailbox on https://portal.office.com you can not send yourself an email to your personal or business domain and (by replying to yourself) get domain email aliasing.
However if you have any regular (free) microsoft account/mailbox (for example @outlook.com or @live.com) you can readily do this.
If you wanted to workaround this problem by logging into your (e.g @outlook.com on https://login.live.com) mailbox
and add another mailbox, the service does not recognize any @btconnect.com account/mailbox, but works just fine for similar @outlook.com accounts.
