Microsoft Quarantining Emails - Constant Contact says 'not our problem'

[HACOC]

I’m dealing with an ongoing issue with Constant Contact (CC) and could use some advice or perspectives. For reasons I can’t figure out, emails sent through CC are being blocked by Microsoft Exchange servers across multiple organizations, meaning they’re not reaching Outlook users.

I’ve asked CC to investigate, but their response has been that I should ask hundreds of newsletter recipients to contact their IT departments and whitelist our emails. To me, this feels like an impractical and inadequate solution. I can’t imagine this problem is unique to my organization, yet CC doesn’t seem to have a process for working with Microsoft or addressing this type of delivery issue.

The frustrating part is that this issue doesn’t occur with Gmail or other receiving organizations—only with Outlook users tied to Microsoft Exchange servers. It’s making me question whether continuing with CC is the right decision.

Am I missing something here? 

For reference:

• Our domain is fully authenticated
• DNS and DMARC settings are in order
• Our domain reputation has been verified as clean
• We’ve worked with a highly experienced web developer

Thank you for your time, questions and/or suggestions!