We are having an issue emailing one particular organization and the only potential issue I've found on our end is we are on the RATS Spam blacklist. https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3alcsbmail.net&run=toolpage
The reason listed at the link above is 188.8.131.52 is listed on the RATS Spam db. That particular ip resolves to 184.108.40.206.bc.googleusercontent.com. It is not listed as one of our mx records or anything in our GMail setup.
We are a Google Apps For Education customer and are setup with MX records for our domain on godaddy.com per Google recommendations. Here's a screenshot of our MX records on GoDaddy. https://drive.google.com/file/d/1n1jpM-VR60_fO0cXf3eH5nldcPKpK47W/view?usp=sharing