SuiteCRM calendar invites are not accepted by SOME mail servers (simply discarded)


I am having problems with some client mailservers refusing to accept our SuiteCRM calendar invites.

Before I continue I have everything setup correctly with respect to my mailservers: WHOIS, SPF, DMARC, DKIM, DNS, AAAA, speed - I have tested my servers with multiple server testing kits, all come back OK (e.g.

Clients mailserver accept other email happily but when we send SuiteCRM calendar invites they are NOT accepted. They are marked accepted/delivered by the receiving mailserver but are then dumped and never send to the INBOX of the recipient.

They are simply discarded without me having any clue which rule I have broken.
When I look at my email logs all I ever see they are sent OK but they did not arrive in the INBOX “Mar 30 10:14:54 mail sendmail[9310]: 22TNEoKB009300: to=HIDDEN, delay=00:00:03, xdelay=00:00:03, mailer=relay, pri=182639, relay=HIDDEN, dsn=2.0.0, stat=Sent (OK TrackingID:3a52a66b-877e-4cbb-a99d-1477df9f301d)”

They are also not in the JUNK/SPAM/WHATEVER box, the mailserver simply discards them.

When any of our clients use any of the following as email providers (security/emailhosting) we cannot send any invites at all to them, they are simply discarded (EVERY other email arrives!)

Has anyone experienced anything like this?
What have you done about it?
Has anybody any idea what I can do about it?