Hi @Dirk Baumbach ,
Welcome to our forum.
Please kindly understand that Office 2007 has actually reached the end of its support lifecycle, meaning there are no new security updates, non-security updates, free or paid assisted support options, or online technical content updates, it's recommended to upgrade to the later version of Outlook to get a better user experience.
Accoring to your description, the issue may be more related to your outlook client, due to the end of support, I couldn't test on the same environment with yours, as I know, the corrupted send/receive file or outlook data file may cause any send/recieve issue, have you tried to test on a new Outlook profile? If not, it's suggested to create a new Outlook profile and readd your accounts to check if the issue has any difference(Control Panel>Mail>Show Profiles>Add>readd your account)
If the issue still exists, it may caused by your corrupted outlook, please try to repair your Outlook client to check.
Hope your issue would be resolved soon.
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