Do you have some kind of intrusion detection with active blocking running on the network? Are connections to the SMTP server restricted (in the properties of the SMTP service)?

Microsoft Corp. Thursday night issued a fix to patch a new vulnerability that could give an attacker user-level privileges on Windows 2000 systems running its Simple Mail Transfer Protocol (SMTP) ...

I'm having to move an SMTP server from a Linux VM using postfix to a Windows 2012 R2 server. So I installed the standard IIS 6 SMTP service in Windows 2012 R2. Everything seems to be working fine ...

Phishers taking advantage of Gmail’s SMTP relay service to impersonate brands Your email has been sent As the number of malware and ransomware attacks continue to become more prevalent, cybersecurity ...