You already have an Active Directory account and you want to create an Exchange Mailbox. You clicked New Mailbox command from Action Pane, selected the account and clicked New. Now you get this very “informational message”, Exchange refused to create Mailbox because it “could not convert property WindowsEmailAddress to type SmtpAddress.”
Do Not Panic. Oh! you already panicked and searched Internet and found this blog. Alright then, let’s interpret this error message and fix this issue.
The error message is saying the existing Active Directory account is set to mail property with invalid e-mail address. Exchange is saying the existing e-mail address from AD account is not SmtpAddress type which supposed to be Name@domain.com.
Solution: Open the AD account properties in Active Directory Users and Computers console and check E-Mail property in General tab. Remove the value from E-Mail property and try creating the mailbox again.