Some outlook users can't reply to my emails using apple mail with Gmail server

Hi everyone

Over the last month, some of my customers can't reply to my emails. The customers are using Outlook and I'm on Apple Mail V14.0 using a Gmail account. They get this error message (attached).

My collegaue has the same issue. He has a Macbook and is using Thunderbird.

A customer said when he removied my signature and any other links and he was able to reply. But this isn't a realistic solution.

I have to use the Gmail account and really want to keep using Mail. I tried using Outlook, which fixed the issue but I just couldn't get on with it. Any tips or soultions would be greatly appreciated!

I'm on Big Sur V11.6, MacBook Air (13-inch, Early 2015)

Thanks in advance