. . . . . "mail server" . . "MTAs" . . . "Message transfer agent" . . . "Message transfer agent" . . "message transfer agent" . . "Messaging" . "email servers" . . . . . . . . "email server" . "mailer" . . . . . . . "mail transfer agents" . . . "messaging system" . "Mail Transfer Agents (MTAs)" . "large-scale e-mail systems" . . "MTA" . "mail transfer agent" . . . "messaging" . . "mail systems" .