Why only some of my users don't get the confirmation emails?

My users are personals from facilities, and they are using their job emails (not their private Gmail, Yahoo…). And it seems that their security system blocking my first-sign-up confirmation emails. So I change the setting to everyone to sign up:

But still, those user get a failure message every time they try to sign up. So I signed them up myself, but still they get failure message…
Anyone had the same problem with organizations’ emails? Is there any advice what to ask from their IT support?

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