how to remove bitbounce?

Hmmm. see if you might have this...

https://chrome.google.com/webstore/detail/bitbounce-gmail-extension/biehoicockplinlnmimmbafdlkpmohcg?hl=en

To find it's parts...

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Use the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck.

So you’re seeing this message sending to one specific user, or with mail sent to all users you’ve been contacting?

If this is bouncing when sending mail to one specific user, make contact with that user through some other means and ask them if they're using Bitbounce for anti-spam, and ask them to whitelist your address. Or pay the fee. Or they might not want to receive email from you.

If this is arising with all users, then there’s likely something else going on here. Post the EtreCheck report as was requested, as a starting point toward resolving whatever has happened here