Postfix: smtp_address_preference

For mailserver/system  administrator :

If your mailserver has problem to connect to ipv6 address (especially if your mailserver doesn't support it) and throw errors something like this:

Dec 10 00:05:57 localhost postfix/smtp[16925]: connect to mta7.am0.yahoodns.net[2404:160:0:f:0:2:42c4:7621]:25: Network is unreachable
Dec 10 00:05:57 localhost postfix/smtp[16925]: connect to mta6.am0.yahoodns.net[2404:160:0:f:0:1:6288:d9ca]:25: Network is unreachable
Dec 10 00:05:57 localhost postfix/smtp[16925]: connect to mta7.am0.yahoodns.net[2404:160:0:f:0:2:42c4:7624]:25: Network is unreachable
Dec 10 00:05:57 localhost postfix/smtp[16925]: connect to mta6.am0.yahoodns.net[2404:160:0:f:0:1:43c3:a8e6]:25: Network is unreachable
Dec 10 00:05:57 localhost postfix/smtp[16925]: connect to mta6.am0.yahoodns.net[2404:160:0:f:0:1:42c4:7622]:25: Network is unreachable

then you may want to check the default smtp_address_preference parameter in main.cf. Most probably it is defaulted to ipv6 which is true on my postfix installation (Mageia). Change it to ipv4 to instruct postfix to look up MTA with ipv4 address only (which is I think supported by all major players like Yahoo!, Gmail and Hotmail for compatibility reason or they will lose lots of their customers) . So:

smtp_address_preference = ipv4

will save you before you consider to upgrade your infrastructure to ipv6. :-)

Have fun!