Would it be possible for reMarkup to strip all zero-width non-breaking
spaces from its output, unless there was more than one in a row (in which
case the number would be decreased by 1)? That would solve the problem.

I see your point.

What are your thoughts on allowing valid HTML entities through to the
browser without further escaping (outside of code blocks)? There seems to
be no reason to have them in text, and they could allow escaping.

What about triple backticks in code blocks? Code blocks can contain
anything, so I consider not being able to put three consecutive backticks
in them a bug.

T6143, while closed, seems to be a real-world use case to me.

@epriestley I hope to fix this in a very general way, which will also solve many of the other tickets related to Remarkup (T6276, T5951, T5427).

I'll take this and T6002. Where in Phabricator's source code can I find the relevant PHP code? Edit: Found it in libphutil.

libcurl supports SMTP (see https://curl.haxx.se/libcurl/c/smtp-mail.html) and can be used instead, provided that the relevant functions are exposed to PHP. This avoids needing to shell out to an external executable.