@Paris:
Thanks for the link. The previous final is 1.3.0, correct ?
No, they weren't spam. I've configured some spam filters in my provider's web interface, but they all are set to move spam to spam folder for 30 days before deleting them. The latest lost mails were just 2 weeks old.
No specific sender, no relation between them.
Yes, I'm using Outlook 2010 on a Win7 64bit laptop. In Outlook the mail account is configured as Pop3 because I never trusted IMAP, so I have at least a backup of my lost mails there. I'm using my devices alone, no one else has access to them. I use this setup together with aquamail on Galaxy Tab and S4 mini since months, I never had any problem with lost mails before.
Regards from germany
Roger