How to send a technical support feedback to Solarwinds?

You had best find out who your account manager is and talk via them. If you don't have one, or don't know who they are, then open a "customer service" support case.

FWIW, the error being generated tells you exactly why you can't reach that email address, so that route will never work.

find out who your account manager is and talk via them

Unfortunately there is no info about the account manager in my customer info, so I have no idea.

why you can't reach that email address, so that route will never work.

Yes, I can read. But wrong configured mail-forwarding (or any other internal stuff) inside Solarwinds  - it's not my problem.
Or if they decided to finally close technical support feedback communication channel, without any notifications - that's sad. We used it often.

>> Yes, I can read. 

Hey dude - chill. I'm just trying to help.

>> But wrong configured mail-forwarding (or any other internal stuff) inside

Why is it wrongly configured? To me it seems they have deliberately done that and at a guess is purely for internal communication only.

>> no info about the account manager in my customer info

That is true, it as far as I can recall has nothing there. My presumption is that you knew who this was. As I previously  suggested, raise a support request to customer service, asking who your account manager is and how you can contact them.

No offence, I just upset with their attitude towards clients.
Case is already created btw

Sadly it is also my experience - but we are of a big enough size that we appear to have some clout and our account manager is very good and responsive.

Good luck.

Sadly it is also my experience - but we are of a big enough size that we appear to have some clout and our account manager is very good and responsive.

Good luck.

Thank you

May I ask you - do your company has a premier or enterprise tier support?