You're right, Morbius. Mailout as a single word is given by some dictionaries, including Dictionary.com and Wiktionary. It can mean either the act of mailing out a newsletter or circular, etc, or the item mailed out. It can be used in relation to physical mail or email.
It appears the mail-out version is used more often, but mailout is also used, as you say.
It will be accepted from now on as a rare word.