Unfortunately, there is no way to be absolutely sure the emails sent to your guests won't end up in their spam folders instead. The nature of the Internet email system originated half a century ago, and was never designed for how it's used today. The best we can do is to work within existing systems, and lower the likelihood that your emails will be viewed as spam.
1. If you're not using the OwnerRez Verified Email system, and an email address from a domain you own, you should do so - it's a straightforward way to significantly reduce the risk of spam identification. Free Gmail accounts are notorious for spam, so you don't want to use those if at all possible.
2. Does the content of your emails look like spam? There are a number of testing tools and resources on the Internet you can use to see how spammy your standard emails are:
https://www.mail-tester.com/
https://blogs.constantcontact.com/how-to-avoid-spam/
3. Many email client programs include the ability to add addresses or domains to a whitelist, that the inbox knows not to flag as spam. You can ask your guests to look into this and add yours. However, every client is different, so they'd have to research how to do that themselves.
4. Sometimes, if your guests simply add your email address to their address book, your messages are automatically accepted.
5. Even with all of the above, anyone receiving any email can flag it as spam manually. If you are getting regular spam complaints, perhaps the problem isn't computer systems in between, it's what your messages look like in the inbox? For example, if you are using many emojis in your subject line, that's a hallmark of spam.
