I have two installations of dovecot 2.18, apart from the necessary changes to allow them to replicate amongst themselves the configs are identical. Both are running Ubuntu 14.04 and are kept up to date. One is running 64bit and the other 32 bit (for historical reasons). Both have ipv4 and ipv6 addresses.

The 64 bit installation allows access on port 143 and 993 on both ipv4 and ipv6.

The 32 bit installation does not allow access on 143 on ipv4, but allows ipv4/ipv6 on 993.

Both installations happily allow connections to any other ports that dovecot might be using on both ipv4/ipv6, as well as everything else that is running (on other ports) on those machines.

There are firewalls, but no warning messages. Switching firewalls off makes no difference.

Tcpdump shows the SYN packets arriving on the (correct) interface.

Any suggestions?

Dirk