You have been sending email to a particular address for a long time. Now you get an error message that reads “Outlook does not recognize the email address.” The email will not send.
To resolve this issue you will need to re-enter the address in the address book. Try sending again with the “new” address and it will go. This is caused by a number of reasons: not signed in properly, incorrect spelling of the email address or the pst file is extremely large and/or corrupt.
Seek assistance with correcting this Outlook problem. As always, before making any changes to the pst file make a backup copy off system to protect your data file.