Greylisting (sometimes spelled graylisting) is a method of defending e-mail users against spam. A mail transfer agent using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will most likely try again to send it later (see disadvantages), at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried, and spam sources which re-transmit later are more likely to be listed in DNSBLs and distributed signature systems such as Vipul's Razor.