Mail from specific sender didn't arrive first time, but did when sent again later

Is there any way to figure out how it is possible that an email wasn’t received in my mailinabox account this morning (sender did not receive any error message), then when asked to send again, the same email from the same address was sent, and arrived with no problem.

Please help me out with any suggestions, where can I look for any clues?

Sounds like the message was greylisted:

Greylisting

Your box uses a technique called greylisting to cut down on spam. Greylisting works by initially rejecting mail from people you haven’t received mail from before. Legitimate mail servers will attempt redelivery shortly afterwards, but the vast majority of spam gets tricked by this. If you are waiting for an email from someone new, such as if you are registering on a new website and are waiting for an email confirmation, please be aware there will be a minimum of 3 minutes delay, depending how soon the remote server attempts redelivery.

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Thanks for your reply.
I checked and though it’s true that it was the first time email was sent from the specific address, but the first email was sent at 10:33AM and never arrived. Only after we asked the sender to resend the email did we receive the second email at 6:18PM.

In this case, could it still be a greylisting problem?
And once we have received an email from this domain, should I still put the senders IP in the /etc/postgrey/whitelist_clients.local file for greylisting?

It could still be greylisting, it depends on whether or not the sender’s server is configured to resend when MIAB rejects the initial message. If the sending server’s IP isn’t already in the whitelist, you could certainly add it.

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OK, I added the senders IP to my /etc/postgrey/whitelist_clients.local file,
then I ran these commands:

service postgrey restart
service postfix restart

Thanks a lot for your help!

Have a great day