It appears that Andrew C Aitchison via mailop <and...@aitchison.me.uk> said:
>On Tue, 17 Dec 2024, Gellner, Oliver via mailop wrote:
>
>> The only validators which I found that correctly reported a problem
>> are Mailhardener ("The selector 'init_dkim' contains an underscore,
>> some SMTP implementations will not accept a DKIM selector with
>> underscores. If possible, choose a different selector without
>> underscore") and Uriports ("The provided DKIM selector is invalid").
>
>Is underscore forbidden or merely not supported by some implementations ?
>
>My reading of RFC6376 doesn't suggest that underscore is not allowed in a
>selector and they are explicitly used in next component of the DNS query.
Look at the ABNF. A selector is a sub-domain which is imported from RFC5321
which says it's letters, digits, and hyphens, i.e., a hostname.
They really are forbidden, any DKIM selector with an underscore is invalid.
R's,
John
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