Why does it matter if I have english characters in my From header?

Why does it matter if I have english characters in my From header?

Hello,

so I'm using web2py and it's mail sending capabilities. I've managed to successfully have the from header to be as I desire using English characters but I get an error when I switch to non-Engilish characters for example:

From: Αβγ < user@mydomain.com>

At some point I was getting a 553 error but now I'm getting a "Connection unexpectedly closed".

Any idea why it matters whether I have english or not characters?

thank you,