Elizabeth. There are lists of email addresses for sale on the dark web (and probably the on the ordinary web). It's interesting that you use BT email as at one time BT used Yahoo for their email services, and Yahoo had a major breach a while ago when criminals harvested a large number of email credentials. (BT started moving users from Yahoo to their own BT mail service in 2014 and as far as I knoiw no longer use Yahoo. The Yahoo breach was in 2013 but didn't come to light until 2016.
https://www.ncsc.gov.uk/news/yahoo-data-breach-ncsc-response ) Obviously, if you've only signed up with BT recently they must have acquired your email address from somewhere else ...
Yahoo isn't the only source - there's been many other beaches: -( Several years ago a colleague and I discovered quite a number of our corporate email addresses on one of these lists. Our best guess as to how they got there was that when people were leaving the company and having leaving drinks they sent an email to a large number of people in the company but also copied it to the private addresses of a number of ex-colleagues, which were with companies like Yahoo that had been breached - the criminals probably just copied all the email addresses in the "to" and "cc' fields and added them to the lists they sell. Similarly, when Plusnet had a breach 20(?) years ago my private domain email address and address book were copied - before then I'd had no spam to my private domain (fortunately me my password wasn't stolen unlike the Yahoo compromise). And should anyone put an email address in the text of this (or any other) forum, which can be viewed by anyone on the internet, then there is likely to be software scanning the internet that can harvest it ...But it sounds like it would be good to attend one of Mark's drop in sessions, probably more hands on and useful than my theorising :-)
Michael Ixer ● 1545d