Update Postfix to the supported version

Postfix 3.10 was released this month (February 2025), marking the end of support for Postfix 3.6.X, which is still used in Zimbra even in 10.1.5.

"Postfix 3.10.0 stable is now available. Postfix 3.6 - 3.9 were updated earlier this week, after which Postfix 3.6 will no longer be updated".

Source: https://www.postfix.org/announcements/postfix-3.10.0.html

I think it's important to update such a critical Zimbra component.