Hostopia and PayPal payments - another PHP mail problem?

[#1]

This is not PHPmailer related, so I have opened a new thread.
We use PayPal to accept most of our merchant services online payments.
Although the payments themselves still go through* following  the move of (almost) all our PHP to Hostopia, PayPal’s IPN and PDT notification mechanisms (which are the only Hostopia-dependent part of the process) now only work sporadically. It appears that PayPal successfully calls the PHP handlers (for IPN and PDT) which we run on Hostopia, but the emails which (among other things) the handlers send out are mostly vanishing. 
In the past three years (pre-Hostopia), every successful payment generated a corresponding IPN notification. Now we are in the weeds/merde/hosed.
Does anyone else have this problem – which looks like yet another PHP mail issue?
* well… they should – the payment flow itself is via a Home Pages server plus PayPal's own backend

[#2]

Are you still experiencing this problem, or was it resolved along with some of the other issues you've reported?