Disable ipv6 sender feature doesn't work

[lucaforni]

Describe the bug

If you disable ipv6

Additional information/context

Check:

In smtp_sender.rb row :
if (@source_ip_address && @source_ip_address.ipv6.blank? && endpoint.ipv6?) || Postal::Config.smtp.disable_ipv6
The last or is wrong and let every mail in soft fail