from `man stunnel`
>
> verify = LEVEL
> verify the peer certificate
>
> level 0
> Request and ignore the peer certificate.
>
> level 1
> Verify the peer certificate if present.
>
> level 2
> Verify the peer certificate.
>
> level 3
> Verify the peer with locally installed certificate.
>
> level 4
> Ignore the CA chain and only verify the peer certificate.
>
> default
> No verify.
>
So it seems you can just add
verify = 0
to your config.