421 validating sender

Whenever we attempt to deliver a message and the recipient server returns a soft bounce, we will retry delivery for up to 8 hours in the following intervals: 10 minutes, 10 minutes, 15 minutes, 30 minutes, 1 hour, 2 hour and 4 hours.Unfortunately this cannot be adjusted and is hard coded in our environment. I just checked mailgun's domain verification and one of the MX was unverified and then I checked it again and it verified. The error that you are seeing is due to the recipient's server either 1). I'll have to wait until tomorrow to see if the retries start going through.First, Office 365 tests each smart host that you entered to help ensure that Office 365 can connect to each of them via SMTP.Second, Office 365 tests the ability to send an email through your new connector.Your letter may have a one address in the letterhead, the address of your specific department, and an altogether different address on the envelope, the address of the institution that your department belongs to.The address that’s on the envelope is known as the envelope sender address when you send an email.

Skip ahead to the section later that corresponds with a failure you're getting.This section provides troubleshooting information for senders who are having trouble reaching users by email.If you are an user looking for support with your account, please visit our end user support page.Smart Screen® email filters are influenced by a number of factors related to the sending IP, domain, authentication, list accuracy, complaint rates, content and more.Of these, one of the principal factors in driving down a sender's reputation and deliverability is their junk email complaint rate.