One day I was informed that the accounting department on one of the computers could not send invoices by email.
There was a big pause during the sending and then a notification was displayed:
Unknown error (SMTP error code 3)
After testing various configurations for connecting to the mail server, an error occurred all the time, according to the logs on the mail server, it was clear that the connection was still being made.
The cause of the error turned out to be the free version of AVG antivirus, if you disable it, then emails began to be sent successfully, so I just replaced the antivirus with another one.