Mail has correct outgoing smpt address, but Comcast will not accept it for the correct email address. Only on one of the computers (MacBookPro Lion)

Question

Always worked until this morning, and works on a different copy of MAIL on the other computer. When I try to send mail using my comcast email address, as always, Comcast rejects that email address for the smpt address that is correct and has always worked before. I can't find any .plist files for MAIL to delete, either.

Answer 1

SERVICE ANNOUNCEMENT
Dear Valued XFINITY Internet Customer, 
Action Taken:
On August 1, Comcast announced that we will no longer support the use of port 25 for sending email. Many older email clients (such as Outlook Express) default to using this port to send email. Please read this message to understand how this action may impact your ability to send email, and what you should do next.  
Comcast Webmail Users:
If you use a web browser to access Xfinity Connect or other webmail service, this action will not affect your ability to send or receive e-mail.  
Email Program Users (Outlook Express, Outlook, MacMail, etc.):
If you use an email program, this action will disable your program's ability to send email until you change your email program settings to send email on port 465.  To protect your email security, click on the link for your current email software, then follow the step-by-step instructions to change your settings.
Outlook Express
Outlook 2003
Outlook 2007
Outlook 2010
Windows Mail
Mail (Apple OS X)
Don't see your email software? Then locate the preferences for your mail account in the software you use and provide the settings listed on this page.  Prefer professional tech support? A highly trained, Tech Expert can remotely change your settings to secure your email. To find out about one-time service and fixes, please call 1-877-550-0793 or visitxfinity.com/SignatureSupport.  If you are not using Xfinity Connect and use another email provider, please contact your provider for its recommended port settings. Most email providers offer an alternative to port 25 for sending email.  If you have additional questions please visit our Help and Support site or our Customer Forums where many questions are discussed and answered by the Comcast community.  Thank you for choosing Comcast!  Sincerely,
Comcast Customer Security Assurance
This is a service-related email. Comcast will occasionally send you service-related emails to inform you of service upgrades or new benefits.  Services and features are subject to Comcast's standard terms and conditions of service and are subject to change.  Copyright 2012. Comcast. All rights reserved. All other trademarks are properties of their respective owners.  Comcast respects your privacy. For a complete description of our privacy policy, click here.  Please do not respond to this email. Responses are not monitored.  This message is for the designated recipient only and may contain privileged and confidential information. Any misuse of this email is strictly prohibited. For information about Comcast email preferences, visit comcast.com/preferences.  Comcast
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Attn: Care