My web hosting service severely rate-limits requests to
xmlrpc.php… which has made using MarsEdit effectively impossible for me for that past five or so years… which practically killed my sites. I went back and forth with their support on multiple occasions during that time, but the problem was never resolved to my satisfaction. Then yesterday—out of nowhere—it occurred to me they were filtering based on the file name
xmlrpc.php so I renamed the file (well, first duplicated, than later symbolically linked) and now MarsEdit has no problem connecting to my site(s).