When you send emails, it may, sometimes, end up going into your recipient’s spam or junk folder rather than his/her inbox folder. It can be annoying as the intended recipients may miss out your important emails.
This can be due to the junk filtering rules configured by the recipient’s email service.
We could not change or control the rules set by the other email services. However, there are a few ways that we can do to avoid our mails from going into our recipient’s spam mailbox:
Reference: Chatterboxer’s Blog
If your emails are still being routed to your recipient's Junk folder, this is maybe because your email address could have been black listed by the recipient. You may request them to white list your email address on their email system.