You need to get into the Service Mode on the copier itself.
- Press the Settings/Registration, quickly followed by 2 & 8 simultaneously, then Settings/Registration again.
- Choose Mode list
- Choose copier
- Choose option
- Choose Network
- Press <Level 1>. It should change to <Level 2>
- Change the value on SMTPTXPN from 25 to 26.
Press Reset to back out of the menu.
You MUST turn off the copier and turn it back on again for the change to take place.