Thank you Patrick for your answer.
But the problem is that the mail that goes out of my system seems to have an envelope sender set to the email of the user writing the bulk mail message. That is at least what the SPF error report says:
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< some.foreign.domain > rejected a message that claimed an envelope sender address of < mailto:user@email.address >.
< some.foreign.domain > received a message from < my.domain > (< ip.address >) that claimed an envelope sender address of < mailto:user@email.address >.
However, the domain < foreign.domain > has declared using SPF that it does not send mail through < my.domain > (< ip.address >). That is why the message was rejected.
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Obviously I have no influence on some.foreign.domain's SPF record.
Is there something wrong on my setup, or is the SPF system flawed?
Regards,
Michael