I have been receiving the following error message a lot when in mid conversation with someone using the /r command to reply back to them.
"Unable to send tell. That person is on your Black List"
I have come to the conclusion that this is caused from a gil spammer sending you a tell after you have already black listed them while you are talking to someone.
I spend a large amount of time using tells to conduct business for sales and synths, and this is rather annoying to say the least. I would like to ask that people on a black list are flagged and there be a priority system put in place where anything tagged with the "BL" flag (Black List) Will be skipped and the game will reply to the last person who sent you a message that doesn't have the "BL" tag.
This is of course just an educated guess and I'm not 100% sure this is causing the problem with the error, but its the best I can come up with.