Hi,
is there a chance to decide, which information, errors are sent to an email.
For example, we are not interested, if someone could not type his/her password correctly, but sure we want be informed if serious problems do occure.
(Or just decide where these information should be logged and/or sent)
I would say that the password example should be logged at most as an INFO message, so one shouldn’t normally receive a message for that (not sure if it’s actually the case, I have to check the source code).
Theoretically one could modify the [handlers] > email section in logging.yaml and add a filter that only leaves specific messages in. The problem is that filters cannot be specified in a declarative way (see this post, for instance). A hack would be to create your own Python module containing your filter’s code and make it available from the virtualenv where Indico is installed.
If someone has ideas for a better way to manage this in the future (while stil making use of the logging module), suggestions are welcome 
The email handler is only for ERROR messages, so unless your Flask-Multipass or Indico version is outdated, the “bad credential” warnings should not trigger an email at all: https://github.com/indico/indico/blob/master/indico/core/auth.py#L105