I swear.  On every Windows Server on every network, I encounter a different weird-ass SMTP issue.  I understand you have authentication, connection, relaying, and routing issues with SMTP that can involve more than the IIS server itself, but it baffles me why every smtp setup is a brand new mystery to solve.  Why can't IIS SMTP services “just work???“

I've setup SMTP on so many servers and today I get a new error I've never seen before:  “The "SendUsing" Configuration Value is Invalid.”

At least a new error made things interesting.  And in the SmtpMail.SmtpServer property is where the mystery always magnifies itself.  I always used to enter the name of the local Exchange server for this value, and SMTP email would work.  Then it stopped working one day, I forget when.  Then I entered SmtpMail.SmtpServer = “”, which made things work again.  But today I use that setting but get nailed with a 550 5.7.1 relay error on external addresses.

SO, HOW IT WORKS TODAY, ON THIS PARTICULAR SERVER, ON THIS PARTICULAR NETWORK, AT THIS PARTICULAR PHASE OF THE FRIGGEN' MOON, is SmtpMail.SmtpServer = “127.0.0.1”, making sure to add 127.0.0.1 as authorized to connect to the SMTP services, as in shown on the IIS SMTP config box below.