Mail in  a Box (mailinabox) can backup its mail with rsync to a destination of your choice. When it was working but your target backup machine has been changed suddenly mail in a box comes with the message: invalid literal for int() with base 10: ”

You checked, double checked your settings and they are all ok .. but still the above message. The reason is that Mail in a Box keeps a record of your SSH keys to protect itself:

messages like:  WARNING: POSSIBLE DNS SPOOFING DETECTED!  and

The ECDSA host key for [my.box.org]:22 has changed,
and the key for the corresponding IP address [target.ip]:22 has a different value. This could either mean that DNS SPOOFING is happening or the IP address for the host and its host key have changed at the same time. Offending key for IP in /root/.ssh/known_hosts:7

Normally this is good behaviour but now you need to have fixed this: its easy with this command
ssh-keygen -f “/root/.ssh/known_hosts” -R [my.domain.name]:port