“Unless you are comfortable with this ending up on a front page of NY Times – don’t put it in wiring” – was one of the first pieces of advice I got at work.
Yes, it’s a bit dramatic, but still great advice. 
10 things you should never put in writing at work:
1.Criticism of a colleague, customer, vendor etc etc
2.Jokes; stories that are derogatory. Stick to “dad jokes” those are always safe.
3.Your romantic thoughts about anyone
4.Frustration with your boss
6.Confidential or proprietary information that you don’t have permission to share 
7.Threats, even joking ones 
8.Discussions of illegal activities (football pool; bets…)
9.Your new job search! 
10.If you don’t want everyone to see it – don’t  write it