Because I care too much about punctuation... Would be really cool to have two adjacent dashes (--) convert to an em dash (—) in the composer. I don't know if other mail clients do it, but it's standard behavior in Slack, Medium, Microsoft Word, etc.
On OS X, this is mostly handled at the OS layer with text substitution rules — implementation would need to mindful of interactions with system-wide rules (OS or e.g. TextExpander).
Because I care too much about punctuation... Would be really cool to have two adjacent dashes (--) convert to an em dash (—) in the composer. I don't know if other mail clients do it, but it's standard behavior in Slack, Medium, Microsoft Word, etc.