I deleted a secondary user from my account. Will people that inadvertently use the email address associated with this (deleted) user be notified that the email account is no longer valid?
If you mean will people sending an email to the deleted address get a "bounce" saying the address is not valid--------------they won't for 90 days. When a user ID (that part of the email address to the left of the @ symbol) is changed or deleted, it is suspended for 90 days, and then released back into the pool of available IDs for any Comcast customer that wants to claim it. But for that 90 day period, the account is still operational-------you just can't access it anymore using the webmail UI. After the 90 days is up, it should generate a bounce message. If you had the account enabled in a desktop client or email app, you would still be able to access it there for the 90 days.