[exim] SMTP transport interface IPv4/IPv6

I have a question about the "interface" option and which IP protocol it
chooses as I obviously don't understand it fully.

Setup:

Host A: Has IPv4 and IPv6
Host B: Has IPv4 and IPv6

Host A: Has a transport with an explicit "interface = 1.2.3.4" IPv4

Host A now sends mail to Host B and uses this transport correctly but
chooses the hosts's IPv6 instead of the configured interface IPv4 address.

"telnet -b 1.2.3.4 Host-B smtp" works fine.

Host B is actually three servers with Round-Robin DNS and IPv4 & IPv6.
The variable $host_address shows the IPv6 address in the debugging
output. The "hostlist" in the debugging output shows all IPv4 and IPv6
addresses.

Any ideas or hints?

Greetings,
Kai Bojens