Hello Mark,
In order for emails to be attached to a ticket, they need to be sent to the service email address with B# in the subject line. So, for example, if your customer wants to add more information to Incident 123 on the service ITHelp, their email needs to be sent to ITHelp@.assist.com and needs to have B#123 in the subject.
If they're responding to the emails we're sending them, this should be done automatically unless they're clearing out the subject to put something else in it.