Seemingly random numbers appear on the SpamSieve icon. What does the number 2873 mean? The numbers come and go by themselves.
This is the number of new good messages since you last activated Mail. However, 2873 is much higher than normal. So if you didn’t actually receive that many new messages, you might want to look into why it’s so high. Did you (manually or via script) apply SpamSieve to lots of old messages? You can use the Open Log command to see which messages SpamSieve has processed recently.