This problem looks like it appeared in email-ext v2.15.
We have configured a "Success" trigger" which, in the "Advanced" section, has all "Send to ..." options DE-selected. Thus if a build succeeds (and the previous build was also a success), then I don't expect any email to be sent. However, since 2.15, on success an email is sent to the global recipient list.
A suggested workaround was removing $PROJECT_DEFAULT_RECIPIENTS from the trigger's list of recipients, so that the list was empty. This initially appeared to have worked, however when Jenkins is restarted, $PROJECT_DEFAULT_RECIPIENTS is added back to list of trigger recipients.
I think the email-ext plugin is confused about recipient lists. There is a system-wide recipient list defined under "Manage Jenkins", then in each job which uses email-ext there is a "Global Recipient List" (which is not system-wide as the name global might suggest), and then for each separate trigger there is a "recipient" list. It is not at all clear how these interact, and what the "send to recipient list" option really means.