Thank you saguaros! Again! 🙂
It is so strange as none of the other JA installs did it in the past or does it now. There is an other fresh GK install what works as usual. Only this is the only one what suffers by this issue. First I thought that Gmail updated their spam filter... but no.
The only difference I see that the first "lost" mails contain this English text: "This is a test mail sent using "PHP Mail". Your email settings are correct!" but the new mails - what just tested now - arrived in the native language instead of the English one.
I did not changed anything caused this. Hmmm.
saguaros