How is it possible to use custom attributes in e-mail?
The sample shows how to use the common ones like {FIRSTNAME} or {LASTNAME}.
If I create a attribute like "attribute_1" and put that key into an mail like that {attribute_1}, "attribute_1" gets displayed instead of the value given.

Hello,

The manual is your friend:
www.limesurvey.org/manual/Email_templates#Email_placeholders

1. It's the same as the other placeholders -> Capital letters. -> "{ATTRIBUTE_1}" resp. "{TOKEN:ATTRIBUTE_1}"

Best regards
Joffm