Error saying invalid email, trying to subscribe to Sourcehut mailing list

I was trying to subscribe Gmane to one of SXMO project mailing lists
just now[0] but received error about invalid email address.

I was not sure which mailing list software to put, I copied 'mlmmj+'
from some other entries at Sourcehut (e.g.,
gmane.org.misc.sourcehut.general), I am not sure if that was part of the
problem or not.

Cheers,
TRS-80

[0] The mailing list email is ~mil/[email protected] but there is
also a web based interface at https://todo.sr.ht/~mil/sxmo-tickets.