Duration of email notifications

[Sebastian Spitzer]

Is there anything known about the fact that the duration of email notifications from Confluence from the publication of a blog, for example, to the receipt of the email notification by the "observer" of the page or blog sometimes fluctuates extremely and sometimes takes 15-300 minutes?

I had an example, publishing a blog on January 5th, 2021 at 2:04 p.m., received the email notification on January 5th, 2021 at 6:58 p.m.
My colleague received the email notification for this blog on January 5th, 2021 at 3:31 pm.

According to our admin of MS Exchange Online, the delivery within Office 365 was prompt. He claims that the delayed duration occurs in Confluence (sending the email notification).

We use Atlassian Confluence 7.3.4 (server)

I would be very happy to receive feedback or even the solution!