I am using WP Autoresponder for my blog subcription service. It had not been working and I did some troubleshooting and the form was not working. I got that fixed and tested it.
I tried signing up through a gmail account--that worked
yahoo account--did not work
and an account based through my main website--did not work.
In all instances the system communicated fine with itself and the subscriber list showed the new sign-ups as unconfirmed, but the email to confirm did not arrive--except at the gmail address.
I've posted at the WP Autoresponder help forum where there was already a thread about this issue, but there are three others with the issue, no one has responded and the first post was 3 months ago.