The widely-read Yes, Virginia, you can use emojis in Marketo Subject lines sets the record straight: as long as you manually Q-encode the characters, there’s no barrier to using emojis in Marketo.
Tons of people use that method with few complaints. But one thing isn’t made explicit (my bad): once you manually Q-encode an emoji, Marketo flips to “manually encoded mode”.
In manual mode, if the Subject line contains other non-ASCII characters, you need to Q-encode those, too. (In normal/automatic mode, as long as Marketo doesn’t strip a character completely, it’ll be Q-encoded for you.)
I’ve found the most common offender is the single curly quote/typographic apostrophe (
’). Every time time somebody claims there “there aren’t any special characters” they’ve forgotten that curly quotes are not ASCII! They want a Subject like:
🅿️ It’s time to park it and get shopping!
So they Q-encode the parking emoji as
=?UTF-8?Q?=F0=9F=85=BF=EF=B8=8F?= but the output ends up as:
Which can make it seem like emojis don’t work. In this case, you must Q-encode the apostrophe as
=?UTF-8?Q?=E2=80=99?= as well. Then you get the best of both worlds: