Form beiung sent to customer but NOT to Admin

I am very familiar with Baforms and am using this component on a large number of websites. On one of them there is a very strange issue I am unable to resolve:
All form copies are being sent to the customers successfully. But none of the forms is being sent to the admin. Therefore it is obvious that PHP mail is working fine.

The option «Send to Admin» is enabled.
The admin email address is valid for sure and is also correctly entered into the admin email field.
The junk folder has been checked.
There is no Captcha plugin enabled.
I tried with different admin email addresses on different mail services.
I tried with PHP Mail and SMTP in Joomla Global Configuration.
Sending a test email from Joomla Global Configuration works fine.
What else can be done or investigated?

Thank you very much and kind regards
Chris

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