I recommend calling Mellanox to get a case opened properly and discuss support options.
In the US, the support numbers on the website are these:
Mellanox Call Center
+1 408.916.0055
Toll-free (USA only)
86-Mellanox (8663552669)
The support@mellanox.com is a valid e-mail address, but it sounds like your e-mails was filtered somehow.
Sending a request to that e-mail address is supposed to open a case - which someone responds to (probably initially via e-mail).
You should also get auto-replies, providing a case #, etc.
If you don't have a contract, the process is for Mellanox to contact you regarding support options.
It seems the e-mails replies from Mellanox are just getting filtered for some reason - maybe in your junk mail box.