Quoting Timo Sirainen tss@iki.fi:
On 4.4.2012, at 15.29, Jerry wrote:
Now I use relay_transport = lmtp:unix:private/dovecot-lmtp. Even if I change this to lmtp:tcp:1.2.3.4:24 it is still only one director.
I don't know if Postfix supports that. Typically people use a load balancer (cluster).
Perhaps posting on the Postfix forum and asking Wietse Venema would be the logical course of action.
I have a guess though, or alternatively a suggestion if it already
doesn't work like that:lmtp:tcp:lmtp.example.com:24
where lmtp.example.com expands to your two IP addresses. DNS server
handles load balancing by returning IPs in round robin and Postfix
handles fallbacking to the second IP if the first one doesn't work.
Dovecot has similar behavior in several places.
Personally I use MX dns entries, lowest mx entry is localhost, then others.