Hello there,
Unfortunately it is not possible for me to select a customfield as recipient of an email.
I tried the following values as the recipient of the email without success: {{customfield_11520}}, {{customfield_11520.value}}, {{issue.customfield_11520}}, {{issue.customfield_11520.value}}
Before that I check with an extended comparison condition (^null$|^$) if the field {{issue.customfield_11520}} ("Testauftraggeber") is not empty.
Error message:
Send email
The email could not be sent because the "To" field is empty. If you referenced a field, it may be empty.
Screenshot: Automation

Screenshot: Customfield

Many Greetings,
Christopher