How are you giving your agents a real email inbox (send + receive, not just SMTP)?
Sending email from an agent is easy; receiving replies in the right thread with attachments is the hard part. It needs a real inbox plus inbound webhooks, not just an SMTP relay, and most hosted options are US-only, which gets awkward with EU data. The thread suggests rolling your own Postfix setup with a parser that matches replies by Message-ID headers, and a small EU VPS that forwards only parsed JSON, never raw mail.