For those of you who are trying to use Mailgun for SMTP with IPB...I hope this helps.
I had been using Mailgun successfully for several years on my domain. After installing IPB in a subfolder in the domain, I configured the SMTP settings to use the MailGun server (TLS & Port 587).
Created a new SMTP credential on MailGun via their dashboard. Copied the password for the new credential to the clipboard and put both in my IPB setup screen. All to no avail. Kept getting 535 authentication errors thrown back at me. Double/triple checked everything. Still 535 errors.
HERE IS THE FIX:
I used the credentials for the postmaster@mydomain.com that were setup in MailGun when I opened the account. BOOM! It worked.
Can't tell you why. Maybe it is just a timing thing where MailGun requires some time for new creds to work...maybe IPB only authenticates properly with postmaster account...I don't know.
But...It works. Hopefully it will work for you if you have the same issue..
=Kevin
Kevin Raybon got a reaction from christopher-w in Configuring MailGun....A pointer that fixed my issue