When you get "invalid server" is because the connection was blocked in port 587 for any given reason. A ping only tells you if the communication is available, not open. I did a ping to your server and I saw the server online, that tells me that the problem is somehow server to server.
The problem could be (without any particular order):
Invalid TLS/SSL support (missing support or not working properly)
Invalid credentials (username/password)
Temporal network block
The yellow box that you see there is the server response command, plain and simple. That's all that Chevereto can know about the problem. You can try enabling debug mode 3 and try to see if there are some other local issue. You should also look at the mail server logs.
Invalid TLS/SSL support,
it seems that the reason of the error was occurred caz of the error of tls support.
I change the mail server and it no longer happen again.
Please change the catalog to Solved.
Thank you.