Hard bounces without reason in transactional mails

Hi,
I have a problem with a small number of recipients of transactional mails. These show up as « hard bounce » in the Log section. Normally there is a reason given for a hard bounce in the details of the log entry, e.g. « 550-5.1.1 user unknown 550 5.1.1 Unknown recipient. ». But in some cases there is a hard bounce without any reason.
I checked the mail adresses with two different tools for deliverability and these tools say that the addresses are ok.

Is there a more detailed log where I can see the real reason for the hard bounce?

Thanks in advance!

Thiemo