I am using Mailgun to send my emails from WordPress, I always have this on the logs
Failed: wordpress@52fluent.com → webmaster@52fluent.com 'Mailgun WordPress Plugin Test' Server response: 550 Verification failed for <bounce+dd882b.7933e-webmaster=52fluent.com@52fluent.com> No Such User Here" Sender verify failed
Any ideas?
You need to setup in Mailgun for each FROM account, do this inner domain section (of mailgun) and then in SMTP credentials, where you will add your login/from account and the password for that account.
Mailgun uses specially formatted Return-Path address to track bounces. But if you use a different service to receive emails this address is not valid until exists on the server and accepts address tags (bounce+something@example.com).
Some info on how Mailgun handle bounces How can the Return-Path header be different than the actual email bounce recipient?
UPDATE: Here is response from Mailgun support
It looks like the e-mail webmaster@52fluent.com address isn't actually setup yet. Is this an actual working e-mail address setup at your host?