Rob Allen:
It was happening on long urls. Given I was also having the "BCCappearing in the headers" issue, I swapped out Zend_Mail and used mail() instead with no problems. It's yet another thing on my list of things toinvestigate more fully when I get some time!
I've worked on the header encoding last weekend, so it uses iconv_mime_encode() for quoted printable. The updated SMTP transport also unsets a BCC header to solve the other issue.
You'd see this in incubator this week. nico
