I oftern consider mailing 50k to an IRL buddy to see if either of us get auto banned though, but i dont think he'd find it funny either way and return the gold.
If there's a huge transfer of money like that, it'll be noted and possibly quietly investigated, but it won't trigger an auto-ban or anything like that. If they saw that both characters were still acting normally and still had most/all of their stuff, they'd likely think it to not be a hack or sale of gold. Also, there might be an upper limit on the maximum amount of gold transferable in a single mail, but having never sent more than a couple of thousand at once, I wouldn't know.
Also, on the subject of the guild bank: the GM was VERY swift, and when I logged over to Lithaella to make my druid a spellthread, I found all the things taken by the hacker sitting in my mailbox with a note from the GMs explaining what had happened and "thanking me for my patience", even though this was not only the fastest they'd ever sorted a situation (that I know of), but also that I'd not been the one who sent the ticket (think it was Woogle) and they'd taken less than two hours to receive the ticket, read it, investigate, act on and deliver the items to me... I'd been fully expecting to have to make my own ticket and wait a few days! Bloody good show, GMs, nice to see that at least one of you is awake on this battlegroup these days.