My emails are soft bouncing to two addresses on the same domain, the rest of the recipients are successfully delivered to.

We send out emails via Marketo to a variety of clients and prospects, but are running into issues with a particular company. We have approximately 900 contacts for this client, but the two contacts that should be getting all of our communications (they both hold executive positions) will experience selective soft bounces. We have looked at the content of all our emails as well as the subject length, size, and send date, and have not seen a pattern that determined when the message will bounce. The contacts that are non-executives have no issue receiving our messages. So far, the remedy this situation, we've:

1) Asked the company to have their IT department whitelist our company's IP address and domain name.

2) Asked them to verify that all 900 email addresses we have on file are correct (a significant chunk of them were coming back as "invalid").

3) Verified the two executives' email and contact info were correct.

4) Set up an additional step in our campaigns that will re-send a copy of the email to anyone who soft-bounced an hour later (we just implemented this as of yesterday).

Have any of you run into this issue? Do you have any suggestions to remedy this situation?

Thank you in advance!
- April