Customer is reporting an error on Undelivered Email List "Not sent: Blocked by mail filter".
The error "Not sent: Blocked by mail filter" means that the sending rate is too high (Similar emails messages are being sent too rapidly).
Go to List > Mailing > Undelivered Emails and check if the From, Recipients, and Subject have the same information.
Check the time interval the duplicate emails were sent. If a high number of duplicate or similar emails are sent at a single time, chances are they will be blocked by mail filter as they are being sent in a spam like behavior.
To avoid having these type of error message on your Undelivered Emails, Simply add a unique identifier to the subject line for each email message generated from your NetSuite account.