Test mail from admin interface rejected

[jandd]

Describe the bug
A mail that has been sent from the admin mail interface has been rejected by my Exim based MTA with the following error message:

2024-03-06 12:30:57 *redacted connection information* F=<info-bounce@devday.de> rejected after DATA: header syntax ("@" or "." expected after "undisclosed-recipients": failing address in "To:" header is: Dev Day Teilnehmer <undisclosed-recipients:;>): "@" or "." expected after "undisclosed-recipients": failing address in "To:" header is: Dev Day Teilnehmer <undisclosed-recipients:;>

To Reproduce

• Login as admin
• send a mail to a recipient that uses an Exim based MTA for his MX

Expected behavior

• Mail is accepted

[jandd]

see https://lists.exim.org/lurker/message/20130508.103815.62972d63.ca.html