From Longman Dictionary of Contemporary Englishmail something ↔ out phrasal verbto send letters, packages etc to a lot of people at the same time SYN send out The department has just mailed out 300,000 notices. → mail→ See Verb table
Examples from the Corpus
mail out• Okay, okay, okay, um yeah I know I had to mail them out.• Are you going to mail that in the mail box out front.• The whereabouts of the other two tureens are still unknown and every time I open the mail I hold out some hope.• It infects Microsoft Outlook, mailing itself out to all contacts in the address book of the afflicted user. 