A collection letter is one method you can use to collect on a delinquent account.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
We are writing to collect a debt. Your account with [Insert name] has been delinquent for [Number of Days] days. We have tried to communicate with you many times via phone, mail, and email to work out repayment for this debt. However, repayment and communication have not occurred.
We do not want to send this debt to collections. Thus, to avoid such a situation, we must have a check for [Amount Due] on or before [Date]. It is important we receive payment as soon as possible or we will submit your account for collection. If you have any questions, please feel free to contact us.