In what order does SMTP virtual server send outbound emails (not FIFO like I thought).

I have configured an smtp virtual server for mail relays. I see mail in the outbound queue; it appears as though the older emails get sent out after newer generated emails. I am trying to determine in what order does smtp prioritize outbound emails. I assumed it was first in first out; clearly this is not the case.
Please advise.
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SRGTAMPAAsked:
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peakpeakConnect With a Mentor Commented:
That depends entirely on the software. There's no rules how. One example: The mail server groups the messages to the same domain in one group, next domain to the next etc. In the meantime it might have a thread that picks the ready groups and put them in the queue, the SMTP engine, running in another thread (or even threads) picks the mails in the queue, in what order is again dependent on the software. Since so many threads are running simultaneously there's no feasible way of telling the order they go out but sending a small text message takes only some milliseconds so in practical terms the order does not matter. My advise is letting the mail server do what it does best: sending emails.
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peakpeakCommented:
Also it depends on the receiving end: if the target mail server does not answer your mailserver will retry a number of times. The target mail server might very well be slow at responding. The swift target email servers will get their mail from you first, then the slower ones, then the retries linger in the queue for a day or two (depending on your settings)
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