Thanks so much for taking the time to review this! > I'd rather not completely replace the old with a brand new code > because that makes it hard for me to see what's really changed. I thought this would be ok, since the existing version is a complete rewrite of the original (so there's precedent for complete rewrite of a function in a commit to fix a bug), and if there were tests showing the behavior to be the same as before, except where this bug is fixed. (Though I see the tests are currently broken). Based on your feedback, and some help from #emacs, I made a patch that is very minimal to the existing code, with better commit message, and attached it here. The patch removes the =(unless (bolp) ...)= guarding inserting CRLF. The RFC says the "boundary delimiter MUST occur at the beginning of a line, i.e., following a CRLF". =(bolp)= is not enough to guarantee the boundary is preceded by CRLF. It can be true when the point is after just "\n". Because CRLF is inserted unconditionally after the =cond=, the code does not include the boundary's CRLF in each branch of the =cond=. > when `filedata` is an empty string, this add an additional \r\n > compared to the current code. This seems right to me Me too, and all the other clients I tested. > I expect the decoding software will skip the \r\n at the of the > header and then look for \r\n, so it's important to have > two \r\n What you said is true. In addition, they also accept "HEADER\r\nfile content\n--BOUNDARY" as the content "file content", and consider the last "\n" as attached to the boundary. That's where the file's final "\n" gets lost if the file's content was initially "file content\n". > There remain some questions on this patch: While fixing this bug, I found a few more problems in addition to the two that you mention here. I was not addressing them yet, since I thought I should fix one bug per patch. > I suspect we can also simply this code by moving the first (insert > "--" boundary "\r\n") before the loop, and the second into the loop > so we can make it insert "\r\n--" boundary "\r\n" (and thus remove > \r\n from the end of each of the preceding branches). Almost, but not quite, or at least not without some awkward (to my eye) repositioning of inserting boundaries, "--", and "\r\n". The final boundary complicates things. It is different from all the others, it is "--BOUNDARY--" instead of "--BOUNDARY" Here's what I ended up with when I tried that, https://git.sr.ht/~ozzloy/emacs-bug-63941/tree/simplify-insert-boundaries-and---/item/mm-url.el#L397 This passes the tests in =emacs/tests/lisp/gnus/mm-url-tests.el=.