Setting up tin for OS/X

I'm returning to tin after a long absence - I used to use it back in
the early '90s.

I'm using OS/X and it's all working well, apart from a problem with
posting. It looks as if this has been encountered often before, but I
haven't found a fix that works for me.

When I try to follow-up a post, it fails because it has my address as
{name}@{machine-name}.local. I've given it my correct e-mail address
and set

disable_sender=ON

in ~/.tinrc

I've also tried setting up /etc/tin/tin.defaults but that doesn't do anything.

What do I need to do?