It could be related to GDPR. Having email address as the login means it is unique. Contact to that email address that may contain bank details will go to the correct email address Some systems don't like emailing an email address more than once with the same email. For DD change notifications...
Didn't say it would work but that would be my guess given the timing. Maybe GDPR as well so one person unsubscribing doesn't affect multiple accounts. Maybe they are moving to a new email marketing company like the one we used that use email address as a primary key making DD price rise emails a...