The problems with sending out a 12,000 recipient email to announce the start of ticket sales to a popular and quickly sold out event are:
a) You get about 500 "out of the office" autoreplies back
b) You get a lot of people replying to the post (a dedicated email account) and asking questions that need to be forwarded on to the client
c) You get to watch the REPLY TO ALL dingbats conversing with friends about it.
That is all.