Do all of your SMTP accounts have a distinctive name/Decription when you view them in outgoing server (SMTP) in account settings) I recommend including the email address they are meant to work with in the description.

Now check the identities for the relevant incoming account that the outgoing server selected for that identity is correct. Your provider used to encourage reuse, but now requires a one to one relationship and that is the root problem for most of those coming to this forum.

I've compared all of the settings on this email account to my other email accounts on Comcast and they all appear identical except for the email addresses. I rechecked that both the email address and password on Thunderbird is the same as what they are if I sign on directly through Comcast. The only difference I see is that there isn't a SMTP line in the saved passwords for this account.

Going back through all of the steps this morning and rechecking everything again and now I'm wondering if this is not the problem. Attempting to log in to this email gives me the message "mail.comcast.net username ***.comcast.net failed. (*** instead of actual user name) and options to retry, enter new password or cancel. Do I just need to actually change my password for this email? Thanks.