> On Apr 1, 2021, at 12:02 PM, Michael Grimm <trash...@ellael.org> wrote:
> 
> 
> But it is good to know that smtp_address_preference might help me with other 
> ISP blocking my IPv4.

For such cases I use the transport table:

  master.cf:
    smtp unix ... smtp
    smtp4 unix ... smtp -o inet_protocols=ipv4
    smtp6 unix ... smtp -o inet_protocols=ipv6

  transport:
    # IPv6 slow or rejected by exampl4.net
    example4.net smtp4
    # IPv4 slow or rejected by example6.net
    example6.net smtp6

This does not require any changes in global defaults.

-- 
        Viktor.

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