Mail : Meaning of 'tilde' sign adjacent my addresses

I am using Mail 16.0 as email program on my MacBookPro, using mainly Gmail addresses.

Attached screenshot shows a 'tilde' symbol appearing adjacent some of the Gmail addresses.

Maybe it indicates a problem with the account??? Can anyone explain what it means and how to remedy?

I have been into Gmail and nominated my icloud.com address as the forwarding address but this still comes up.

The problem doesn't appear to happen with my iPhone; I thought they were synchronised and therefore the problem would appear with both.

Thanks.