If the reasoning is about DNS SRV being a standard: Autoconfig is already a quasi-standard today, supported by over 10 email clients and many servers and server-side software, and we're currently in the process of standardizing it as IETF standard.
Bug 342242 Comment 76 Edit History
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If the reasoning is about DNS SRV being a standard: Autoconfig is already a quasi-standard today, supported by over 10 email clients and many servers and server-side software, and we (i.e. the people at the IETF and me) are currently in the process of standardizing it as IETF standard.