Gmail assigning Message-IDs with two different domains

I am trying to troubleshoot a mail delivery issue that seems to be related to message IDs for messages purportedly sent from a single Gmail address, to a domain whose mail is handled by G-Suite. Some of these messages are being delivered and some are not, and the things that stand out are:

• the messages being delivered are all initial messages in a conversation, and despite coming from a Gmail address, have a Message ID that is of the form *@hxcore.ol
• the messages that are being dropped are replies in threads, from the same address, but have a Message ID that is of the form *@mail.gmail.com

Unfortunately I am unable to speak with the sender to ask what is differing, but G-Suite support has guessed that perhaps some of these messages are being sent from an aliased account, or a different email client app, which would explain the variation in Message ID domains. My understanding is that this value is typically set by the sending app, or the first mail server that the message passes through.

What I've done:

• I've spoken with G-Suite support, but they're not exactly sure what's going on here. It does not appear that these messages are being flagged/identified as spam or are being dropped for that reason
• I've searched for this strange Message ID domain: when searching for hxcore.ol, I can find a handful (< 100) of results, all from other Message ID fields in mailing list conversations. I can't find any reference to this being an app, or a mail service, or a valid domain (and .ol does not seem to be a valid TLD).
• I've tested my own Gmail and examined outbound mail headers: my Message ID headers all seem to be of the form *@mail.gmail.com, except when a 3rd party service is sending mail on my behalf
• I've skimmed RFC2392 and RFC5322, the latter which states, in part:
  

  Though other algorithms will work, it is RECOMMENDED
  that the right-hand side contain some domain identifier (either of
  the host itself or otherwise) such that the generator of the message
  identifier can guarantee the uniqueness of the left-hand side within
  the scope of that domain

Can anybody shed any light on any of this? I'm trying to understand why initial messages and reply would have different domains on their Message IDs, and why that would affect deliverability on G-Suite.

Edit, 7 months later: Still no clear answer, but from a reply below I've found that HxCore appears to be a framework that Outlook for Mac uses. Presumably this could indicate that these are emails being sent by Outlook on behalf of a gmail account?