Email spoofing alerts in Google

Hi All,

I'm new to this forum.  We use Zendesk as our way for clients to raise tickets to us.  Recently I notice that if I reply to the user within Zendesk, to their email they used to raise the ticket, the reply is getting sometimes flagged as spoofing.  If I send them an email from my Google account to theirs without Zendesk, it does not get flagged as possible spoofing.  Anyone else seeing something like this?  I'm trying to find a full message header from within Zendesk but unable to do so.  Thoughts?

Thanks