One Contractor Not Receiving Validation Email

Summary: We have one contractor that is not receiving the validation email when trying to apply to a job on the external career portal. It is working for all other users. I have had a working session and verified that they are entering the right email address and still not receiving the email. I can see under messages that the email was sent.

Any thoughts on what may be causing this one person to receive the email when all others' do?

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Version (include the version you are using, if applicable): 23C

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