Generally emails are processed based on the order in which the mails are arrived at the Service Desk Manager maileater process.
If the following 2 emails are sent out at the same time, but a mail server takes time in sending out the Email 1 (due to attachments or some other reasons), and Email 2 is sent first to the mail server where Service Desk Manager connects to and received by the maileater process, in this scenario, the maileater will process Email 2 first.
Email 1 sent with some attachments.
Email 2 sent with plain texts.
If there are multiple emails in the inbox, Service Desk Manager will process in order of time it arrives at the maileater process.
If the following 3 emails are received and stored in the mail box after the previous mail check,
Email 1: Received date/time 10:31 18/Jul/2017
Email 2: Received date/time 10:32 18/Jul/2017
Email 3: Received date/time 10:33 18/Jul/2017
Email 1 will be processed first by the maileater, then Email2 and Email3 will be done.