If you have a valid, unexpired license, but ExQuilla refuses to accept it, the most common issue is that the licensed email does not appear in the list of valid emails. Here's how to check that:
Go to the license page (Tools/ExQuilla/ExQuilla license options...) and look at the line "Valid Emails". That line should contain a list of the emails that are currently in use with ExQuilla. So for example, I have the entry "email@example.com" listed there.
If the email does not appear, there are these two main possibilities:
1) If a different email appears, then you may have specified an incorrect email for your license. If so, please add a support ticket here, stating the correct email, and we will generate a license for that email.
2) If no emails occur, then there is something not working in your ExQuilla account configuration. The emails will only show up after you have successfully connected to an Exchange server, so the server URL must be correct, and the user information must allow you to successfully connect to that server. If you have not connected to the server or setup your Exquilla account, please setup the account (Tools/ExQuilla/Add Microsoft Exchange Account).
If no emails occur in the list, but you seem to be connecting successfully to the server, then there is something wrong with the identities configured for the account. Go to Tools/Account Settings/(Your Exchange Account)/"Manage Identities ..." You should see one or more email there, with the first email matching your licensed email. If not, try to correct the emails in that list to include your licensed email.
If nothing seems to work, the best option is usually to delete your ExQuilla account, and then add it back. Feel free to file a support ticket where with details of any issues you have.