Outgoing SMTP server just started refusing to send with error "Mailbox has not yet been accessed"

Yesterday morning (19 Sep 2021), all was working fine. Today, however, I am unable to send any mail through Comcast's outgoing SMTP servers at smtp.comcast.net on port 465 with authentication. I have changed no settings, but now both my mail client application and the SSMTP program on my system are unable to send mail through Comcast's outgoing mail server.

Here is the protocol log of a failed attempt to send mail:

09:46:26.891 *** connectToHostEncrypted: smtp.comcast.net:465
09:46:27.662 <<< 220 resomta-ch2-06v.sys.comcast.net resomta-ch2-06v.sys.comcast.net ESMTP server ready␍␊
09:46:27.662 >>> EHLO localhost␍␊
09:46:27.699 <<< 250-resomta-ch2-06v.sys.comcast.net hello [73.119.255.56], pleased to meet you␍␊
09:46:27.700 <<< 250-HELP␍␊
09:46:27.700 <<< 250-AUTH LOGIN PLAIN XOAUTH2␍␊
09:46:27.700 <<< 250-SIZE 36700160␍␊
09:46:27.700 <<< 250-ENHANCEDSTATUSCODES␍␊
09:46:27.700 <<< 250-8BITMIME␍␊
09:46:27.700 <<< 250 OK␍␊
09:46:27.700 >>> AUTH PLAIN␍␊
09:46:27.985 <<< 334 ␍␊
09:46:27.985 *** [sending authentication data: username '##REDACTED##']
09:46:28.383 <<< 235 2.7.0 ... Authentication succeeded␍␊
09:46:28.384 >>> MAIL FR​OM:<##REDACTED##>␍␊
09:46:28.569 <<< 450 4.1.0 <##REDACTED##> Mailbox has not yet been accessed␍␊

As you can see, the authentication is accepted, but then the server refuses the MAIL FROM command with an error code 4.1.0, stating "Mailbox has not yet been accessed."

It looks to me like a new policy has been implemented, requiring users first to authenticate to the incoming mail server before they may utilize the outgoing mail server. (This is reminiscent of some email service providers in the 1990s.) If indeed Comcast's policy was changed yesterday, I was not made aware of any such change. The new policy is unreasonable, as I do not use Comcast's incoming mail services, and Comcast does not permit me to use any outgoing mail servers other than smtp.comcast.net, so now I have no means of sending email.

If the policy change was unintentional, then someone goofed, and the server configuration needs to be fixed.