I had the same issue. I suspect it has to do with the size of the file, as I had the same thing happen with another very long story ( I think it was "Six Times a Day").
...and then I go back to check which story I compared against, and discovered it actually was about 10x smaller than Three Square Meals (11,184KB). So I'll have to see if I can find another story to compare against in the epic story length grouping.
Edit: First attempt was actually Reasonable Bounds, at 7471 KB. Which also triggered a direct download. Suspecting the issue here is likely to be e-mail agent that is restricting file size. If I had to guess, it's probably chucking anything over 4MB (4,000K), just need to find a story just above/below that line to test with now.
Edit 2: Potential scratch on that initial theory. Just tested one at 4,262 KB (With the strictest definition of 4MB being 4,194,304 bytes, that knocks down a potential 4 meg cutoff) and it e-mailed without issue.
Edit 3: And we have a winner. Tim, The Teenage MC @6973 KB.
Error! Emailing the ebook has failed: SMTP Error: data not accepted.SMTP server error: DATA END command failed Detail: Error: message file too big SMTP code: 552 Additional SMTP info: 5.3.4
Wolves and Dragons of the Blood: Revelations, the next smaller story according the advanced search, will send to kindle without incident, at 6728 KB.
Lost & Found the next larger story at 7425 KB simply goes to direct download.
Edit 4: Note for Laz. I downloaded and then e-mailed the mobi file SOL provided (for Three Square Meals) to Send-to-Kindle(using Gmail), and Amazon appears to have processed it, so the size restriction is imposed on your end, and could be raised, if your mail server will let you.