Info at this link may be helpful: https://forums.businesshelp.comcast.com/conversations/connectivity/solved-smtp-server-port-auth-used-to-work-no-longer-does/615e0154dc872674e0fc3bd0 quote: I talked to a knowledgeable tech support person at Business Class today. She said that Business Class no longer provided outgoing SMTP service, and I would need to log in to Xfinity.com and set up my web email, then use smtp.comcast.net. It turns out that because we subscribe to Xfinity TV, I happen to have an Xfinity account. That may not apply to others. It's possible that Business Class has its own web email, I don't know.

But still it did not work; authentication rejected. After more searching, I found the answer hidden away deep in the link tree of how to set up desktop clients. Xfinity has recently implemented additional security which requires each user to enable access to their email account from third party applications. Without this enabled, no access, incoming or outgoing, is possible from anything but the Xfinity web email. I had to go to my newly set up Xfinity email, click on Settings (gear wheel), then Security, and enable the check box for access by third party applications.

Now my smtp.comcast.net email forwarding works from the Linux/postfix setup described above: smtp.comcast.net, port 587, my Xfinity (not Comcast Business) email and password as account name and password.

See additional info - make sure this is selected: https://www.xfinity.com/support/articles/third-party-email-access

SMTP Server Name: smtp.comcast.net Port: 465 with Connection Security: SSL/TLS OR Port: 587 with Connection Security : STARTTLS

Authentication Method: Normal password User name: full email address - note the user mentions this is the Xfinity (not Comcast Business) email and password. Basically this sounds like you would be using an xfinity account to send emails for the business email address which you have set up in xfinity as an allowed forwarding email address.

Dear Toad-Hall,

This is an amazing post AND IT worked! I've been on the phone for the past two weeks with both Comcast Business and Xfinity and no one let me know that: Turn Your Access to Third Party Email Programs On/Off in Xfinity Connect

To provide our customers with additional security, we added a new checkbox to the Xfinity Connect, which may affect your ability to manage your Comcast.net email through third-party programs like Outlook, Google, Apple Mail, Thunderbird and more.

These programs could expose your Xfinity ID and password to fraud and other risks, including the potential for external programs to read, download and delete emails on your behalf.

Comcast email security settings

Once, I did this, voila I could send out emails again!! Why Comcast couldn't let me know what to do rather than tell me that I needed to change my email address to xxx@comcast.net, I don't know.

Thanks to your post, Toad-Hall, we are back in business! Have a great weekend.

Sounds like a case of comcast/xfinity updating their requirements and not actually informing their support personnel. Good to hear all is up and running again.

somehow it is working now but i had to reinstall theunderbird. Because i am using comcast through Thunderbird it may have been a comcast issue