SymptomsMerak mail server is used with a third-party antivirus software. The antivirus is configured to scan the outgoing mail only, but for some reason the incoming mail is also scanned for viruses.
ResolutionUnfortunately, this is a feature common to all mail servers that do not support mailbox-level filtering (that includes SmarterMail and Merak mail servers), and, thus, cannot be avoided. If a mail server doesn't support mailbox-level filtering, incoming mail will be always checked for viruses.
The only exception from this rule is MailEnable mail server which supports mailbox-level filtering and, thus, gives you an option of avoiding the scanning of the incoming mail. All other mail servers currently supported by Plesk do not support mailbox-level filtering.