I have an old RH5 email server ( I know, but it's hard to update because we have an uucp email account with the ISP )
When a Windows PC client tries to send mail through smtp, the server sends a SYN to port 113 (AUTH), normally replied by RST from the PC, as Windows does not understand auth. XP SP2 firewall breaks this, as the flow starts from the server and I just want to turn this useless protocol off. Disabling auth in inetd.conf does not work.

Disabling auth in inetd turns off your responses to identd - it doesn't stop your mail server from sending out that auth request.

What mail server are you using? You will likely need to change the configuration for that.

Wow! Redhat 5. I take it the smtp system your referring to is based on sendmail. If so, I don't remember if the redhat 5 supplied sendmail rpm's disabled the ident timeouts by default.If not, you can disable the ident timeout in one of two ways:

1) In sendmail.mc, just add
define(`confTO_IDENT', `0')dnl

then create a new sendmail.cf and restart sendmail
# m4 sendmail.mc >sendmail.cf

2) If your redhat 5 system does not have all the m4 tools loaded to create a new sendmail.cf, then edit sendmail.cf directly (not the preferred method) and either add or change the following line to:

O Timeout.ident=0

FWIW: The later redhat sendmail rpm's now set the ident timeout to 0 by default.