Hi,
That is correct the message you receive back is the default failure message from the text_api.
Disabling the mailbox will stop SDM to fetch the corresponding.
You may also want to disable the corresponding pop3/imap mailbox as your users may continue to send emails to it thinking this is a valid one.
1/ they will no know that no ticket is created for them.
2/ emails will accumulated there as SDM will not process and delete those anymore.
Hope this help.
/J