We are standing up a new data center and users received SMTP errors while testing. We were able to ping the server so DNS lookup and network traffic was working. But we only had a generic SMTP error message. So, I fired up telnet and tested our SMTP connection using these commands:
telnet
o smtp.psadmin.io
ehlo
mail from:ps@psadmin.io
rcpt to:user@psadmin.io
data
Test Email
.
When we first tested send emails through the SMTP server, we received an “Unable to relay for user@psadmin.io” error. That’s much more descriptive and a useful message to send to the Exchange administrators.