The support article titled “In-Game Mail Delay” (for World of Warcraft) has no mention of mails between Real ID or Battletagged friends.
I thought it might be quite possible that mails with attachments between such friends were instant, similar to between guildmates, so having no mention of Battletags or Real ID at all left me still wondering.
Could you please add a line that mentions whether such mail is instant or not?
NOTE: I am not including single characters on a single other character’s friends list in this, only Battletag and Real ID friends. The original character-to-character friend mail with attachments seems obvious enough that its not instant, imo.