check out your outbound messages, every recipient has that add in their device.
One of N recipients of your email outbound from ####.ieee.org has a viruses, Trojan or malware installed on their Computer or mobile
As the number of recipients increases so does the chance of email adress list harvest.
We trace most of our spams to business emails sent to clients in Asia, e.g. Chinese,HK, Malaysian and Thailand clients have little infosec and a nasty habit to cc internally to 3..10 unknown others in their firm.
Easy solution....ask IEEE to change your adress and drop the contaminated one.
Bon courage
Jon