Deskpro looks at the email headers to determine who the user is. Some email server configurations do not send the correct user information in the From: header. This can be a problem if you are automatically forwarding emails or using distribution lists.
Examine the headers of an email message that was affected by the problem. Sometimes, the correct user address is retained in the Reply-To or X-Original-From header, even if From: is wrong.
You can tell Deskpro how to prioritize these headers in Admin > Tickets > Settings, in the Email Settings section. Use the handles at left to drag the headers into the order you want, putting the most reliable header at the top.
If the problem is only with user emails forwarded to the helpdesk by your agents, and the agents are ending up as the ticket user instead of the correct user, the helpdesk is not detecting that the emails have been forwarded. See this article for how to fix this.