The time you spent at any job is precious, regardless of your position or reasons for leaving. Even the worst job with tedious tasks and ignorant bosses serves as a platform to learn more about the business world and move on with your career a little bit wiser. You may love the idea of leaving your old job behind, especially if you're off to a new, exciting position with a different company. However, saying goodbye to your co-workers can be emotional. You've spent hours with fellow employees most days a week and you've formed a bond.
Write each letter differently depending on who they're being sent to. Letters to co-workers who you've formed a good friendship with can be impersonal, lighthearted and lengthy. Letters to co-workers who you're merely acquaintances with can be shorter and more formal. You can even send a bulk e-mail to all of your co-workers, stating that it's your last day and wishing everyone luck, then follow it up with personal e-mails to select people.
State that you're at your last day of work and mention something valuable that you've got out of the job. Let your co-workers know what your next career step will be and how they can reach you once you leave the company -- providing an e-mail address is enough, but you can give close friends your phone number as well.
Avoid being overly emotional if it doesn't come naturally. You may think you should be emotional about leaving your job, expressing feelings for each of your co-workers, but you should aim to be genuine in your farewell. False emotion could ed up making your former co-workers uncomfortable. Try to be heartfelt without being mushy.
Send your farewell e-mail on your last day of work, when you've finished with your tasks for the day. The end of the day is the best time to say your goodbyes. After you send the e-mail, you may have co-workers stopping by your desk to wish you their best in person. If you're in the middle of work, you'll have trouble finishing up for the day as well as giving your co-workers the attention they deserve.
As much as you may want to, avoid saying negative things about the company, your co-workers or your boss. You may need your current job as a reference in the future, even if, right now, you have a new job lined up. Don't assume that negative information won't reach back to management and negatively affect your career in the future.