email safe word means in slang: A word that is usually unrelated or unusual in context to the topic of an email thread can be used to modify the subject of the thread. The participants may also agree to end the thread if the thread becomes too inappropriate. So that everyone on an email thread knows that the topic is too personal, and they don’t feel shameful about it.

Similar to a safe term in BDSM.
(in Slang Dictionary, added by Kaelyn Nixon)