The message "A mailbox couldn't be found for email address" typically indicates that the email address you are trying to access does not have an active mailbox associated with it. There could be several reasons why this is the case:
Inactivity: As you mentioned, if you haven't logged into the email account for an extended period, the email service provider may have deactivated or deleted the mailbox due to inactivity. Different email providers have different policies regarding account inactivity, so it's possible that the account was closed or marked for deletion.
Account closure: It's also possible that you or someone else with access to the account closed or deleted it intentionally. If you no longer have access to the email account, it's worth considering whether anyone else may have taken action on it.
Incorrect email address: Double-check that you are entering the correct email address. Small errors or typos can prevent you from accessing the mailbox.
To clarify the status of the email account and understand why the mailbox couldn't be found, you should reach out to the email service provider's support team. They can provide specific information about the account's status, including whether it has been closed, deleted, or is still active.