What causes delayed sending time in Outlook?

One of our customers is experiencing a delayed delivery for E-Mails send out of a campaign to an outlook mailbox although Eloqua shows the same sending time for all participants in the campaign.

After reviewing the campaign and the participants there is no obvious explanation for this delayed sending behaviour.

As it was an internal campaign to inform employees and some employees reveived the mail imediatly and others had several hours delay we came to the conclusion that it is no firewall or windows defender problem.

Does anyone know what Eloqua settings could cause this problem?

Thank you very much