Plugin offers double opt-in only. However, with some small modifications, you can add single opt-in support. I wrote about it here: How to add single opt-in to WP Autoresponder
Not recommendable for a sequence of autoresponders: I cannot recommend the plugin. I put quite some time in it to make everything run. I have only one use case: Send out a sequence of autoresponders. However, it is completely unclear when these emails get send out. If you set a sequence of 0, they'll send more or less immediately (with the next wpcron-event). If you set a sequence of 1, well, I don't know. Something like "after 24 hours + X" or so. It is NOT "on the next day". It was too much of a hassle to understand at what time the next autoresponder is sent, so I uninstalled the plugin.
Troubleshooting I figured so far:
For sending out mails: Make sure to setup the SMTP server correctly and note that new subscribers get wp-cron scheduled. Use cron view or something similar to check out whether your cron tasks are scheduled or not.
Two more possibilities for errors via SMTP:
Does your server use encryption? Can you connect to the SMTP server without a certificate warning?