Does Xfinity Block Spam Before it Reaches Me?

... Does Xfinity decide for me what is spam without giving me a chance to see it? ...

There are several possibilities. The main ones are:

1. Yahoo may decide the message violates one of their rules for email and does not accept it
2. Yahoo may consider a message spam and does not forward it
3. The sender may have included a "do not forward" code, so Yahoo doesn't send it out
4. Comcast/Xfinity may decide the message violates one of their rules for email and does not accept it
5. Comcast/Xfinity may decide the message is spam and delivers it to the spam folder
6. Comcast/Xfinity may decide the message is OK and delivers it to the Inbox folder

"Override Security Filters" only operates on #5. It doesn't affect the other possibilities.

... Is Xfinity's spam filter so good that I don't have to worry about it accidentally blocking non-spam? ...

Hardly. They sometimes miss obvious spam, and sometimes "spam" mail that is clearly isn't. 

It's a problem . . .

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