There are several strong reasons why people might wish to send email messages anonymously. First of all, a message passes through several systems or stages in the process of delivering. In any of the stages your personal message can be copied and correspondingly read. Moreover, your e-mail is backed up and stored as a part of systems. One can easily retrieve information monitoring traffic through your e-mail hub or ISP (Carnivore/DCS1000). It is very disturbing when somebody is watching you, even if you have nothing to hide.
The second reason why you might want to communicate anonymously using anonymous messages is to prevent disclosure.

Send an anonymous compliment:

You're a great friend

I’m so proud of you

You do a great job at the office

Everybody loves to be around you

Your new haircut looks fantastic

You're absolutely lovely

You're amazing

Send some anonymous love and romance:

Can't get you out of my mind

I wish you were single

Leave my boyfriend/girlfriend alone

Send an anonymous message providing some constructive criticism:

Your jokes make other people uncomfortable

The office is not the place to talk about personal matters

Try not to criticize so much in meetings

You are always late

Your negative attitude is starting to impact your co-workers

Send an anonymous message ranting and raving about:

Idiots like you should not be in charge

Your barking dog is driving me nuts

Your company is destroying the environment

I'm sick of your behavior

Mind your own business

Send an anonymous prank or joke:

You're my baby's daddy

The boss wants to talk with you

Sorry about your car

I saw what you did last night

I saw your wife/husband flirting with your firend

Send an anonymous suggestion:

People are noticing your body odor

Don't be so shy, speak up in meetings, you have good ideas

You’d better watch your kids more closely

You've got a beautiful smile

Send an anonymous message looking out for someone else:

The boss is monitoring your Internet use

Beware of your co-worker, they can't be trusted

You are sharing too much personal information at work

People are talking about you

You are dating the wrong type of person

Send an anonymous complaint:

Your service is terrible

My time is as important as the doctor's

Clean up your yard

Stop popping your chewing gum

You’d better hire employees with brains

Anonymously blow the whistle on some wrong-doing:

Officer, I want to report

Teacher, my child told me

Boss, someone's cheating the company

Neighbor, someone visits your wife when you're away

Send an anonymous email intervening in someone's damaging behavior:

We think you may have a drinking/drug problem

You smoke too much

You are getting too thin / fat

Your gambling is hurting you and your family

Send some anonymous gossip:

Guess who broke up/ got caught cheating

Guess who is pregnant

Did you see the news