App Store Connect Invitations - Only received by gmail users

I have three test users. Only the user on gmail has received the invite. One of the others is on iCloud and the other Outlook. I've resent the invites over multiple days and these two have yet to get the invitation. Checked spam as well and they're not there.

Any remedies? Test Flight status is green

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I could see invitation sent toast message but the mail is not actually received. Even gmail account also failing

Happens also for me. Invited many new users but no one received any invitation email.

Not receiving any invitation e-mails either, gmail or not.