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)?

Many users have reported to be getting a long error message saying: Send test e-mail message: The connection to the server was interrupted. If the problem continues ...

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 ...