We are using the professional emailing service Mailgun to send the Payster reminding emails. It's unnecessary to do some complicated setup with DNS settings etc., and this solution works out-of-the-box. We focus on maintaining a high-quality rating for the sending domain "mg.payster-app.com". There can be displayed this notice on the recipient side in some clients, for example, Gmail:

But don't worry; your sender's email address is still the primary and always visible.

You can get rid of this message by changing your sender's email address to {your-shop-name}@mg.payster-app.com. Then set the reply-to email address to your regular email address.