I have had a btinternet.com account for 15 years or more. I am an officer of a voluntary organisation which has its own server. Officers have generic email addresses with a format like "email@example.com" (dummy address) and any emails arriving on the organisation server are then automatically forwarded to officers' private email addresses, in my case to my btinternet.com email address. For example: a member sends an email to "firstname.lastname@example.org" and my organisation's "orgname.com" server then forwards it automatically to "email@example.com".
This has worked fine for several years but about 2 months ago I suddenly got reports that I hadn't replied to emails. We investigated and found the emails were arriving OK on our organisation's server but when forwarded to my private btinternet.com email address were being blocked.
My webmaster tells me:
"I tested out your email and have been sent an error report for non-delivery and it shows that BT Internet are blocking forwarded email because the intermediate server does not match the sending server. The error report is accompanied by an automated suggestion of adding an SPF record but in this scenario it is not possible. The delivery failures are due to BT’s anti-spam policies.