As pointed out on Reddit [1], if you want to trivially see companies using Mailgun it's as simple as looking at:

https://securitytrails.com/list/mx/mxb.mailgun.org

https://securitytrails.com/list/mx/mxa.mailgun.org

[1] - https://www.reddit.com/r/sysadmin/comments/14vtszl/comment/j...