TYPO3 Core

After further investigation, the @ symbol should be allowed by TypoScript parser.
https://github.com/TYPO3/TYPO3.CMS/blob/11c11675e62a8bdef428db63af291185fe35d4fc/typo3/sysext/form/Classes/Controller/FormFrontendController.php#L158

Also, the configuration needs to be:

plugin.tx_form.settings.formDefinitionOverrides.contactForm.finishers {
    0 {
        options {
            recipients {
                test@test\.de = Name
            }
        }
    }
}

The dot in the email address, has to be escaped, otherwise it won't work, as the dot notation of TypoScript kicks in. This is especially bad, if the email address is i.e. defined as a TypoScript constant without escaping the dot. I couldn't figure this out yet.

This won't work:

plugin.tx_form.settings.formDefinitionOverrides.contactForm.finishers {
    0 {
        options {
            recipients >
            recipients {
                test@test\.de = Name
            }
        }
    }
}

Clearing recipients in TypoScript will not work, because here the array is merged and the default value of the YAML form file is still in the array:
https://github.com/TYPO3/TYPO3.CMS/blob/11c11675e62a8bdef428db63af291185fe35d4fc/typo3/sysext/form/Classes/Controller/FormFrontendController.php#L158

Setting

recipients = __UNSET

The following code variation works.

YAML form definiton:

finishers:
  -
    options:
      recipientAddress: default@example.org
      recipientName: Default Name

TypoScript setup:

plugin.tx_form.settings.formDefinitionOverrides.contactForm.finishers {
  0 {
    options {
      recipientAddress = special@example.com
      recipientName = Special Name
    }
  }
}

Yes, this works for TYPO3 9, but the YAML configuration changed to "recipients" in TYPO3 10.

https://docs.typo3.org/c/typo3/cms-core/10.2/en-us/Changelog/10.0/Feature-80420-AllowMultipleRecipientsInEmailFinisher.html

@Raphael, just had the very same challenge and came up with this solution that doesn't involve writing a custom finisher: https://gist.github.com/kitzberger/7a53d517b5991769783fd7326062e157

It's basically using the "variants" feature that got introduced with TYPO3 9.4

Thanks a lot @Philipp, you saved my day! Your solution works great.

But it would be nice if the typoscript version also would work again (specially if options via data e.g. TSFE:fe_user|user|email will be used). This was my solution before:

kontakt_1 {
finishers {
0.options {
recipientAddress = CASE
recipientAddress {
key.data = GP : tx_form_formframework|kontakt_1-15509|singleselect-1
default = TEXT
default.value = email@domain.tld
1 = TEXT
1.value = anothermail@domain.tld
2 = TEXT
2.value = anothermail2@domain.tld
}
}
}
}

The misconception of ext:form with this TypoScript is having a "living value" on the left side as identifier. That's not good design.

It would be better to do something like this:

recipients {
    10 {
       email = test@test.de
       name = John Doe
    }
    20 {
        ...
    }
}

As such, I'll keep this issue open, and still think this is an issue in ext:form.

However, for the parser issue: Since v12, with the new TS parser, identifiers can contain `@`. I'll add a patch that adds a dedicated test for this as proof.