答案1
你必须transport_maps在你的main.cf
#/etc/postfix/main.cf
#...
transport_maps = hash:/etc/postfix/transport
#...
#/etc/postfix/transport
[email protected] smtp:smtp.a.example.com:25
[email protected] smtp:smtp.a.example.com:25
example.com smtp:smtp.b.example.com:25
transport_maps (default: empty)
Optional lookup tables with mappings from recipient address to (message delivery
transport, next-hop destination). See transport(5) for details.
Specify zero or more "type:table" lookup tables. If you use this feature with
local files, run "postmap /etc/postfix/transport" after making a change.
参考：Postfix 文档