New member invitations automatically declined

I’m trying to invite new members to collaborate on my project, and for some of them, all belonging to a specific email domain (a client company of ours), I automatically (~30 seconds post invite) get an email titled “Invitation declined”, stating in the body:
‘’’ has declined your invitation to join the Itai Bar Sinai / Mona python client project.

Inviting users from other domains work fine.
Any ideas?