Fyi is commonly used, even in professional communication, to indicate that a message or a part of a message is for informational purposes only and doesn't require any action. ... In formal situations, and especially in email subject lines, it's a good idea to write fyi in all caps—it looks a bit more professional.

Is it unprofessional to use FYI?

“FYI” is just rude and can easily become a tool in passive aggressive communication when forwarding an email from someone else – “FYI, you should know about this”. ... Make your intention clear so that the other person doesn't start to question the hidden meaning of “FYI”.

Why is FYI rude?

"FYI" is quite often used in offices, and elsewhere, for quickly sharing relevant emails with coworkers. So perhaps the writer used it for speed and it did not occur to him/her that, in the context, the message might sound rude to the recipient.