Error 1045 (28000) Access denied for user 'USER'@'REVERSE_DNS'

[Claudio Basso]

Hi all,
when connecting from remote machine (from any remote machine) to my MySQL server,
I receive this error:

ERROR 1045 (28000): Access denied for user 'MyUser'@'hostxx-xx-xx-xx.contoso.com' (using password: YES)

hostxx-xx-xx-xx.contoso.com is the reverse dns lookup of the remote client

Mysql 5.5

What I've already tried:
- Activate / deactive skip-name-resolve variable
- Created user with host = %
- Created user with host = IP
- Created user with host = the reverse dns record
- Tried with --protocol=tcp argument from client

The strange thing is that also if I activate the skip-name-resolve directive,
I receive the same error, but if I understand correctly, if it's enabled the server must not do the reverse lookup, I'm expecting an error like 'MyUser'@'MyIp'...

Any help?
I've tried for several days tons of solutions, no one works...

Thanks
Claudio