E-mail etiquette: what not to do

Just imagine that a colleagues asks for a service supplier's phone number just in case something urgent needs to be done while you might be absent. So imagine you promise your colleague to e-mail the phone number.

Just imagine this hypothetical situation.
According to the e-mail etiquette you should not in that case e-mail the phone number in the body of an e-mail without further explanation....to that service supplier himself.


If you would do so, you might receive a polite but very confused e-mail in return from your supplier after which you might start to laugh out loud into tears after which you might need to explain your error to everyone around you in the office.
...in that hypothetical situation of course.