Postfix & client IP reverse DNS

Did you tried STFW? I'm not postfix fun (I'm consider it as loozer's MTA), but nothing more than 3 URL from Google

smtpd_client_restrictions =
  check_client_access hash:/etc/postfix/maps/access_client, ...

Content of access_client

IP  <policy>

You can do it as Lasy Badger suggests. Here a bit cleaner:

Modify main.cf:

smtpd_client_restrictions =
  check_client_access hash:/etc/postfix/client_access

Create /etc/postfix/client_access:

00.00.00.00    OK

After that you have to postmap /etc/postfix/client_access and then postfix reload.

But note: It will not work! At least not in your case. Because your "client" uses a dynamic IP. So you have to edit it every time your client changes his IP. Probably once a day.

So note that it is not your fault how you configured your server. It is the fault of your client to have a bad mail server configuration. One should not run a mail server behind a dynamic IP. Running one behind a dynamic IP looks like being a Spammer.