If you are sending your mail with Comcast's webmail and you have the newer Open-Xchange "Appsuite" webmail ("connect.xfinity.com/appsuite/..."), please see "How do I change my name?" under "Preferences" on https://connect.xfinity.com/appsuite/apps/faq/index.html.
If you are sending your mail with something else, what is it?
@Anon890921 wrote: ... recipients of my emails see me as "xxxxxxx.last_name@.net," instead of firstname.lastname@example.org.
Double check your UserIDs on your home system with the lookup tool at https://idm.xfinity.com/myaccount/lookup. After you enter the requested information you'll see a list of the UserIDs on your account.
If the email address (UserID) is itself ".last_name@", you'll need to have Comcast fix it. If an employee doesn't take care of this for you here, Comcast Security Assurance might be able to help. Give them a call at 1-888-565-4329 (from http://constantguard.comcast.net/help/report-abuse/). Note that this is NOT one of the general customer service numbers. The first-line reps there are sometimes not very helpful, so you may need to be persistent and keep at them until you get a satisfactory answer.
Please post your results!