Some email addresses are changed to be “via Groups.io” because of the DMARC email authentication policy. Certain email services, such as Yahoo Mail and AOL, have set up their DMARC profiles so that Groups.io has to change all their users' email addresses to the “via Groups.io” form; otherwise, their group messages would bounce.

Note: This situation is not limited to Groups.io. All email group services have to deal with it.