From Longman Dictionary of Contemporary Englishbulk mailbulk mailTCMif you send something bulk mail, you send large amounts of it for a smaller cost than normal → bulk
Examples from the Corpus
bulk mail• Spammers regularly extract all the email addresses from Usenet to add to their bulk mail databases.From Longman Business Dictionarybulk mailˌbulk ˈmail advertisements and notices about services that companies send out in large quantitiesSend your bulk mail with our quick and efficient service. → mail 