Actually, I've experienced this on and off over the last few years and it may be the email's content. I just had this happen last week.
Client sends me a link. "What do you think of this company?" I reply. Never received, no bounce. Sometimes it DOES bounce, with message "refused" but not always.
After various frustrating attempts to re-send, I begin modifying the content . . . . lo and behold, when I remove the link they originally sent, it goes through fine.
This prompts several hard tests, send a different link, it's fine; send just the target link, no go. The link itself is in mail filters for whatever reason, probably a result of spam filtering.
Some key mail servers are hotmail, yahoo, msn, aol . . . and I'm sure there are others.
My dedicated server mail passes through a relay server at the parent servers, and even some of those never get past the relay server for the very same reason (walked through this one with support.)
So the previous post could very well point to the same issue, something in the content, and very likely a link to a site that's in their filters.