Not receiving ingoing emails from some domains !!

[ltward]

What is in your smtpd.cfg file?
You can omit posting all the comments here; they make no difference.
grep -v ^# smtpd.cfg > /tmp/somefile
Then post the contents of /tmp/somefile here

[ltward]

Things look good at your end.

Do all sending domains have this problem, or is it only senders from iberdrola.es, correo.aeat.es, and perhaps a few others?

I tried telnetting to your MX host on port 25 and doing an SMTP conversation and had no troubles.
I also did "sendmail -v yfahssi@islgrupo.com" and had no problems.

So I am thinking the problem might be on the senders' servers.
Can they find your MX record?
dig islgrupo.com mx

Can they telnet to port 25 on your MX server and send you a message?
ehlo myhost.mydomain
mail from: <me@here.com>
rcpt to: <yfahssi@islgrupo.com>
data
(some test data here)
.
quit