I have been testing out the plugin before I use it on my site. I have a shared hosted site on Godaddy, and am using the latest version of wordpress.
I did try the following with both SMTP on and off.
When I send the test mail or newsletter things work fine. All is good.
But as soon as I try to use the forms for subscription, no emails go through. So I do not the confirmation mail (Double opt-in is enabled), nor do I get the activation email if I try to Resend it manually in the subscriber list.
I have the following form code:
[newsletter_field name="email" label="Your best email"]
[newsletter_field name="first_name" label="Your name"]
[newsletter_field name="downloadList" number="2" hidden="true"]
UPDATE: Emails appear after a long time on my custom domain, email@example.com. But still, no emails show up on .yahoo.com and .gmail.com domains.