I have a btconnect.com email address. Using Windows Live Mail on a windows 7 laptop, and also a windows 10 desktop. The laptop has stopped sending, but still receives ok. The desktop still works both ways. Have tried different settings in the outgoing smtp server, but to no avail.
Windows 7 and live mail is not supported anymore by us or Microsoft. You may still be able to get into the webmail via secure.business.bt.com and logging in with the email address in question. Once logged in follow the email path and it will take you to the webmail section.
If you have any issues please feel free to reach out here