If the recipient is also on Google Workspace, it's almost definitely because their organization's Workspace admin has configured a content compliance rule to add that text to the subject line of incoming messages. See https://support.google.com/a/answer/2364580?hl=en#zippy=%2Cstep-specify-what-happens-if-expressions-... for how that feature works.
If the recipient wants to pursue this, they'd need to talk with their organization's Workspace admin about why your messages are getting flagged this way. As you can see from the documentation above, the content compliance rules are quite flexible, giving admins lots of power in terms of rejecting, quarantining, or modifying incoming messages. Perhaps the admin has chosen to tag any message from outside their domain as "[Sender Not Verified]" or maybe they have some internal verification process. Or something...
If the recipient isn't on Workpsace, there are fully-equivalent features for Microsoft 365 and most other email server offerings.
Hope that helps,
Ian
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