Thank you email not getting sent via SMTP

Hi,

I have a problem with Sendy, no matter how I subscribe the user (API or POST request), the thank-you email never gets sent. Note that I use Sendgrid SMTP and not Amazon's SES but that shouldn't change anything.

What works:
- Sending newsletters to subscribers
- Sending test messages

The only thing that doesn't work is the thank-you email. Btw, I'm using single opt-in.

Thanks,

JoeDRL