I’ve got your point. So, I just misunderstood the expunge command, my bad.
The attached picture shows what is meant with “Email connect via Exchange” in Gmail and Outlook.
In SOGo the mails disappeared as well, yes. There are currently 340 mails in my primary mail inbox starting at 07.07.2023 but last week there were mails until 11.06.2023 so another month is gone over the past 5-6 days.
The storage usage is 0.24% of 10gb in use atm.