Office 365 event invites not arriving

We recently migrated a client from an in-house Exchange server to Microsoft 365.  Everything works great, except for one user having issues with event invites.  When they send an event invite to multiple users only some of them receive it.  However when they just email the same people they get the message successfully.

User is on Outlook 2013 and Windows 7
tampatechmanAsked:
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Vasil Michev (MVP)Commented:
Do a message trace on both ends, see if something funny pops up.
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tampatechmanAuthor Commented:
If I add 6 people to an appointment, 3 people get it (randomly).  They are on the sent message in the Sent Items folder but the other 3 that did not receive the message are not.
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Vasil Michev (MVP)Commented:
Run a message trace as detailed here to make sure it reached the recipients on O365 side: https://technet.microsoft.com/en-us/library/jj200712(v=exchg.150).aspx
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