Adobe Campaign / deliverability question: we had an email sent to a particular company's employees get sent to the spam folder, and they said the subject line was blank. They all use Outlook.

Any idea why the subject line would get removed for this particular company (or if it was classified as spam)? No special characters, just text.

As far as the deliverability, I believe we just need to work with the company to whitelist us, as that was a common practice before we migrated to Adobe Campaign, so I'm not concerned by that as much as why the subject line was blank, and could others be experiencing this as well. ReturnPath shows the subject line working in all email clients.

Thanks in advance!