While this post is mostly pointed to customers of Arigato PRO, it can be used as guide for everyone who has problems with delivery rates of their outgoing newsletters and auto-responder emails.
Here are two very important steps to take that will greatly improve the delivery of your emails:
Use SMTP. PHP mail works… sometimes. It sends emails but doesn’t construct the envelope too well and generally isn’t too reliable. Setting up SMTP is really easy and takes few minutes. Every paid hosting service provides email accounts with SMTP access. Ask your provider if you are not sure about this. If you are not happy with their quality and delivery rates you can use affordable services like Amazon SES, smtp.com, SendGrid etc. Heck, you can even use Gmail as free SMTP service.
Please do use SMTP, it greatly improves delivery.
Use sender address from your domain name. For example if I send you newsletter from this site, it will be sent from email@example.com. If I use domain name different than calendarscripts.info this will reduce my success a lot. Many email servers will block the incoming email due to domain mismatch. In many other cases the email will end up in the spam box.
If you are using SMTP account from other domain (like Amazon SES, or Gmail) your sender’s address must be the same as the SMTP account. So if you are sending from SMTP firstname.lastname@example.org, use the same as sender address.