First verify that the VIP used for outgoing traffic has a correct reverse DNS record:
Fix it if necessary. Next, change the
/etc/mailname on every VM. Value should be strictly identical to the reverse.
Last step, indicate to every MTA’s VM to use a custom EHLO value instead of the hostname. For exim simple create a file
/etc/exim4/exim4.conf.localmacros with the following content: