What is the MTA retry timeout on your servers?
The retry timeout setting defines how often the MTA (Mail Transfer Agent) will try to deliver messages that cannot be delivered at the first attempt.
SiteGround's servers use Exim for Mail Transfer Agent. If a mail cannot be delivered at the first attempt, Exim tries to resend the message every 15 minutes for 2 hours. Then the retry intervals are increased, starting at 1 hour and increasing each time by a factor of 1.5, up to 16 hours. Then Exim retries every 8 hours until 4 days have passed since the first failed delivery.
This is the default setting on our servers. It cannot be changed on the shared servers due to their specific setup and security policy. Changing the retry timeout is possible only on our Cloud Hosting and on our Dedicated Servers.
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