I had multiple case of having an e-mail few hours after it was sent by the sender to me.
It is not my client because it is configured to refresh like the other account on my thunderbird and it works fine for all of them.
Do you have any suggestion of how can I debug this problem ?
You can look at /var/log/mail.log to see if it gives you any ideas, e.g. were there any delivery attempts that were rejected prior to the final delivery? Otherwise it would be hard to know without access to the sending server.