We had a problem at work with receipts and support request replies to all our customers on a certain popular free email provider that also had a big ad business (but was not Google or Microsoft...) getting filtered out as spam. We'd contact them, and they'd say they would fix it, and in a few days the mail would start going through--for maybe a month then it would go back to getting blocked as spam.

Word of this got to our guy who handled ad buying. He called our ad rep with them, and asked pointedly "why the fuck am I spending $X thousand dollars a month to buy ads from you to get new customers, and then you are blocking those new customers from getting their receipts and support replies???".

The ad rep put our guy on hold for a minute, then came back and said he had their head of IT on conference. He asked the head of IT that question. The head of IT put us all on hold for a couple minutes, then came back with a couple engineers who worked on their email system on the line, and told them to whitelist us so that nothing of ours could be blocked as spam, and promised it would be done in something like half an hour.

It was, and we never had a problem there again.