I also tested your "headers: subject: ..." section under "email_configs".
This is what Thunderbird showed me for a "resolved" E-mail:
[image: resolved.png]
Therefore, it all works as expected as far as I can see. Did you remember
to signal to alertmanager that you'd changed its config? (Send it a HUP
signal)
Do note that you included an extra space before and after the words "DOWN"
and "UP" in your template. Hence the E-mail subject has a leading space:
Subject: UP
^^
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