Forwarded email bouncing at Gmail with error 550 5.7.1

The following error message say that Gmail checked your forwarded email and found it to be SPAM

If the email-sending domain's SPF record does not specifically allow yourdomain.com as an intermediary forwarder, then such forwarded-email will fail SPAM check and bounce. As it is not possible to control SPF record of 3rd party email senders, the correct way is to change your email-collection process to comply with current Anti-SPAM systems.

We recommend that you stop email auto-forwarding at yourdomain.com, instead at Gmail goto Settings >> Account and Import >> Check mail from other accounts >> Add a mail account and setup email-collection via IMAP by Gmail from yourdomain.com. As Google says at https://support.google.com/mail/answer/175365?sjid=16510488372932699960-AP#users and I quote

Email forwarding for Gmail users

If you forward messages from other email accounts to your personal Gmail account, follow the recommendations in this section to help ensure messages are delivered correctly:

• Set up IMAP or POP in your Gmail account: IMAP lets you read messages on multiple devices, and messages are synced in real time. POP lets you get messages on a single device, and messages aren't synced in real time. Instead, they're downloaded and you decide how often you want to download new emails. For detailed steps to set up IMAP or POP with your Gmail account, visit Check emails from other accounts.
  
  
• Mark or unmark spam messages: When Gmail incorrectly marks a message as spam or phishing, fix the mistake by following the steps in mark or unmark messages as spam. This helps Gmail correctly identify spam and legitimate messages in the future. 
  
  
• Update your Gmail settings: If you forward messages from another email account to your Gmail account, Gmail might incorrectly mark some messages as spam or phishing. To help prevent this, add your non-Gmail address to Gmail’s Send mail as setting. For detailed steps, visit Send emails from a different address or alias.