You have setup email forwarders in your control panel to deliver emails @yourdomain.com (your actual domain) to Gmail, and they are being returned as undeliverable, or end up in the spam / junk folder at Gmail.
Why this happens:
When spam is sent to one of your email addresses @yourdomain.com, your mail server forwards this spam to Gmail. Gmail then mistakenly thinks that you are the actual sender of the spam, not the original sender. Repeated forwarding of such spam results in delivery problems of legitimate email. This may also be due to your account not having SPF or DKIM setup, or a combination of all. SPF and DKIM can both be created in your control panel under the "Email Authentication" icon.
1) Get rid of your forwarders and use only real email accounts @yourdomain.com or use your Gmail address directly.
2) Go to your Mail settings and Accounts tab at Gmail and add the address you are forwarding from to 'Send mail as'. This is a new feature from user requests, where Gmail will detect that you forwarded from that account and and may help prevent problems such as this.
3) Use Gmail to host your email (without forwarding) by pointing your MX records to them. This method requires that you sign up for Google apps. Once you sign up they will provide you with the instructions for setting up your MX records. Once you have these instructions, these modifications can be done in your control panel.
Alternate solution: (Not Recommended due to false positives and other possible problems)
1) Set up Spam Assassin in your control panel to rewrite subject lines of email that it determines to be spam. The new subject line should contain the word "SPAM" in it to bypass google's check.
For more information, please see: https://support.google.com/mail/answer/175365?hl=en