SMTP error and problem with connection to the service provider server in the same time

Hi, I often get a error when sending mail. Note: Outgoing server (SMTP) servername.com failed. At the same time, I cannot connect to the server admin panel via the browser and the address servername.com. After some time error it disappears and I can send e-mails and connect to the site. When I disconnect the wi-fi connection and reconnect, it also no problem. But problem come back. I checked the internet provider and server administrator. Everything is OK on their part. To make it not too easy, another computer working on the same network have a smtp connection at the same time. It looks as if the lock was occurring on my computer. Just why every now and then.

PING to the server go through during the failure.

The problem was also found in OSX Catalina.