How do I set up PostFix to accept incoming messages only from the EuropeanMX servers?

Create a file /etc/postfix/access and add the following content:

antispamcloud.com OK

Execute the following command:

postmap /etc/postfix/access

Add the following content to the file /etc/postfix/main.cf

smtpd_client_restrictions = check_client_access hash:/etc/postfix/access, permit_mynetworks, reject

Or

If you have already defined smtpd_client_restrictionsin /etc/postfix/main.cf, add the following at the beginning of your definition and replace permit with reject at the end: 

"check_client_access hash:/etc/postfix/access"

Reload the Postfix configuration by executing the following command:

sudo postfix reload

Restart PostFix.

/etc/init.d/postfix restart

Add the following content to the file /etc/postfix/main.cf :

smtpd_restriction_classes = EuropeanMX
EuropeanMX = check_client_access hash:/etc/postfix/EuropeanMX, reject
smtpd_recipient_restrictions = check_recipient_access hash:/etc/postfix/protected_destinations, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination

Create the file /etc/postfix/EuropeanMX with the following content:

antispamcloud.com OK

If you also send and receive internal messages, then you must also add the localhost and localdomain to the file in the following format

domain.example OK
localhost OK
localdomain OK

Create the file /etc/postfix/protected_destinations and add the domain(s) you want to restrict in the following format:

example.com EuropeanMX
example.net EuropeanMX

Postmap both files.

postmap /etc/postfix/EuropeanMX
postmap /etc/postfix/protected_destinations

Reload the Postfix configuration by executing the following command:

sudo postfix reload

Restart PostFix in order to apply the changes.

/etc/init.d/postfix restart