Tried sending a few large mails yesterday (1x 4MB, 1x 5MB, 1x 8MB 1x 11MB, 1x 12MB and 1x 13MB)
Got a few return errors.
Then I split up the larger emails into two parts (e.g. 12MB-> 2x 6MB) and tried to resend them.
It took ages... after about 20-25 minutes I got again errors (0x800CCC6C) and...