It depends how you want to restrict it. I'm not sure whether those are the mail relays you're talking about or the sending addresses.
You can use the check_sender_access directive within an appropiate smtpd_*_restrictions. It's normally best practice to apply all sender, host checks etc. within the recipient restrictions (i.e. after the client has sent 'RCPT To:' )
e.g. to allow only mail from senders @gmail.com and @hotmail.com ...
set smtpd_recipient_restrictions to the following:
Now /etc/postfix/access should be of the form:
use postmap hash:/etc/postfix/access to create the hash table.
Relay hostname or IP
The format of client_access is similar:
Reading your logs
The following is a full excerpt from my mail.log for an example message. I picked a message and got the queue id - 31AF4761F3. It will be in the headers of the mail as well as your mail log file.
$ grep 31AF4761F3 /var/log/mail.log
Sep 4 09:30:38 cutoffs postfix/smtpd: 31AF4761F3: client=russian-caravan.cloud9.net[w.x.y.z]
Sep 4 09:30:38 cutoffs postfix/cleanup: 31AF4761F3: message-id=<007B93C54F154113B36026A22D5E0106@gaby>
Sep 4 09:30:38 cutoffs postfix/qmgr: 31AF4761F3: from=<firstname.lastname@example.org>, size=4225, nrcpt=1 (queue active)
Sep 4 09:30:39 cutoffs postfix/pipe: 31AF4761F3: to=<XXXX@XXXX>, relay=spamassassin, delay=1.4, delays=0.19/0.01/0/1.3, dsn=2.0.0, status=sent (delivered via spamassassin service)
Sep 4 09:30:39 cutoffs postfix/qmgr: 31AF4761F3: removed
You can see in the first line, we have client=russian-caravan.cloud9.net (which is the mail server that sends mail for the postfix mailing list) and the IP address is in brackets. You can use the hostname or the IP in the access file but remember if they have multiple mail relays or ever change their mail relays, you'll need to figure that out.