Gmail SMTP Mail Message error

Source: Send SMTP Mail Message

Message: The SMTP server has unexpectedly disconnected.

Exception Type: MailKit.Net.Smtp.SmtpProtocolException

@bashii

Was the activity working before and suddenly stopped working?

Try to ping the SMTP mail server from command prompt and check if the ping is successful.
If the ping is successful check if the port is blocked or not using telnet.

Eg :
ping smtp.gmail.com
telnet smtp.gmail.com 465