| SMTP Mail delays are usually caused when one of the transient servers is overloaded and the queue becomes too big.
One of the numptys here once decided to do a marketing spam mail and ended up sending 2000 e-mails of 120k each (240Mb). We only have a 2Mbit connection here so it took 16 minutes to send them all, assuming full bandwidth usage. Of course, in reality, my packetshaper will shape that back so it was probably only using 200k so blocked the mail server up for about 5 hours.
Also, there's something called the retry loop. If, for whatever reason, a mail server cannot connect to the next hop, it puts the mail into the retry loop.
Usually, as a rough guide, this is similar to the following;
Try. Wait for 1 minute, Try again. Wait for 4 minutes, try again, wait for 4 minutes, try again, wait for 8 minutes, try again....
and so on and so on. The time between attempts rises every other time until it gets to 24 hours in some cases.
Most servers will try for about 3 days to pass the mail on before it bounces it with an undeliverable.
Also, with Virus filters and spam filters now there's quite a few bottlenecks on the internet for mail.
You also have to remember that compared to 12 months ago, there's approximately 100% more e-mail traffic on the internet.
Mark.
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