SMTP & Relay Issue

I have been using SendGrid and have experienced a variety of issues: Adds characters, messes up html formatting, delay in sending emails, ....

I tried to see if I can use SMTP Server instead.  On a new Azure VM, Windows 2012 Datacenter R2, I have installed SMTP.  I tried to send out an email from a classic ASP page and I get the error "550 5.7.1 Unable to relay for ...."

Any suggestions as if and how this can be fixed?

Thanks,

Esmail Sadeghi