Advice on when to send a x

I recently heard this item on the radio: A German did not understand that when the English put an x at the end of a letter or a text, we are sending a kiss.  They thought this is how we ended all letters so when they wrote to the tax office, (HMRC), they sent the tax collectors kisses. So, we don’t end formal, business letters, emails or texts with an X, just to our friends. And its pretty standard now to finish messages to our friends with a x.