Same issue as @WayneSchulz: tests fine via Comp Maint but get NTLM error via PO Printing. So similar to Wayne I changed the SMTP Encryption from 'NONE' to ""TLS/STARTTLS"" and that resolved it. Also, the client had mentioned that when the error was occurring for a group of PO's, some PO's e-mailed successfully but some didn't, of which I said that seems a little strange because typically with a Paperless error like this, all will fail. ?