I went to add an emoji to a subject line (within mailflow rules) only to discover that one of two emojis I was trying to use "wasn't supported". This is an emoji that's been around for years, so it suggests that Microsoft is pretty far behind on Unicode support within the platform.

Upon opening a support ticket, I was informed that most aren't compatible, which made absolutely no sense to me, since I can use them all within my mail client. This suggests the mailflow rules are in dire need of an update.

The Emoji in question that I was attempting was U+1F4DE (telephone receiver).