An email was "blocked" and returned.  I am a frequent sender to AND
frequent receiver from the email address that apparantly shows the
block.  I do not know if it's a problem on my end or not.  How do I
get a sense of whether this was an intentional block or something I
need to "fix" on my end?

Something similar happened to me. I found out that my ISP was blocking
all email from Europe. Their reasons are still obscure, but they
claimed that the email was coming from addresses that were also
sending spam. At any rate, they gave me a workaround, and I resumed
getting my mail.

I have had emails returned and the reason has been stated.
These were emails which included text like "send this email to 6 of your friends".

Cheers