Every email we send from Salesforce shows up in our clients, prospects, etc email inboxes with one of these long strings:

 via lw7vxw382z3ooluh.ysr5j3mz574hjqmy.6gjky.e-jcdumao.e.bnc.salesforce.com 

We've received multiple comments about this as it looks suspicious even though it's just a cosmetic issue. Is there any simple fix to this?

Thanks