Adobe Sign - Email bounced and could not be delivered

Hello. I am hoping someone can help. I am a user of Adobe Sign within my company. Most esign requests I send to other company employees go through fine and can be signed. one or two addresses though don't get the email. It looks like they don't even get sent out of Adobe Sign. Or IT department say it is nothing to do with the recipient email settings, or the filters. I have tried eveything I can find on here to fix the issue. (Adding echosign and adobesign as trusted addresses, changing filters etc) but it just does not work. It has to be something wrong with Adobe Sign. Can anyone help please?