When you send emails to guests from OwnerRez -- whether automatically on direct bookings, triggers, autoresponders, or manually via Email This Guest -- there are some certain requirements to minimize filtering by spam filters. First off, the OwnerRez email is only for transactional email between you and a specific guest, related to an inquiry, quote, booking etc. For newsletter blasts to many guests at once, we recommend a dedicated newsletter tool like MailChimp -- use the Email List report to dump out a mailing list you can import into any of those tools.
Another of the big requirements is the sending email address. By default, when we send email it'll come FROM your OwnerRez (@inquiryspot.com) email address with a REPLY-TO set to your account email.
We have to do this for spam purposes -- if we sent directly FROM your email address without being authorized, that'd be a spam flag.
This accomplishes the underlying goal -- when the guest hits reply the message will come back to you -- but it can show some "on behalf of" headers in the guest's email client.
To fix that, you need to own a domain and use an email address at that domain. If you own your own domain, say it is acmevacations.com, you can set DNS records that verify us as a sending server. Once you do that, we'll switch over to sending directly FROM your email address -- say email@example.com. Of course your domain will be different, and you can pick any prefix, like your name.
With a web mail provider like FastMail or GMail, they don't authorize us to send so we have to use the REPLY-TO method.
If you've got a domain and email address, contact us and we'll send you instructions from Postmark, our email provider, and get you all set up on the verified sender program.
On a related note, you set up an individual email address per property or property group using the Settings > Theming area. The verified sender process is the same for those as well.