Google has very strict policies when it comes to incoming mail and any spam-like or abusive behaviour can result in your IP getting listed on google RBL. And the results may be that the mails sent through your contact form doesn't reach you or worse, your clients not getting the mails sent from your end.
Here are the common reasons, which can get you listed on Google RBL:
Change in volume of mails to google : A sudden change in the volume of mails sent out from the server either due to spammers exploiting your domain or contact forms
It might be due to the weak password of your email accounts associated with this domain or computer compromised.
If your domain sends a lot of Spam emails to spam trap addresses, this can cause a domain or IP blacklist at Gmail. Also the user complaints (they can simply mark an email as Spam for them) and other issues trigger Gmail’s blacklist filters.
Poor email practices, like forwarding unnecessary emails to Gmail accounts can cause email blacklist.
You are using a new IP address to send emails.
You have incomplete or incorrect DNS records.
Failing Email Authentications
If your mail IP is blacklisted in Gmail, you will get the bounce back with the error codes, "421 or 550" when sending mails to Gmail accounts. If so, you need to contact directly Google team to submit a removal request. You can submit a delist request from the below link:
You should give the blacklist removal team as much information as possible to decide them you are not a spammer.
Before that, please make sure that your account is free from spamming.
Gmail expects you to use authenticated email. Your email should have these DNS records:
Also strictly follow the policies mentioned at : Gmail’s Sender Guidelines