Sometimes, when I send messages to those I know, or when I copy
myself, I get the following in the subject line:  [SPAM]xxxxx  where
xxxxx is the subject.  The most recent one was when I sent out wedding
announcements to about 30 people.  Why is this?

It may be that an innocently used word or phrase in your message is
triggering the filters of spam monitoring software.  You might be
interested in the answer I gave to a related question:
http://answers.google.com/answers/threadview?id=145129

I reckon it is probably being triggered by the email being
sent out to what your ISP/mail server considers to be too
many recipients.  Try breaking out your mailing list into
two smaller groups and see if it still gets tagged.

denco-ga

It sounds like the ISP at the recipients end is running SpamAssasin.
To be given a  definite answer as to why your mail is being marked as
[SPAM], you'll need to get hold of the headers of your mail from the
recipient:

In Outlook Express:
Right click on Message in Inbox list
-> Properties
-> Details
-> Copy and Paste everything from here into a clarification request

In the headers, it should tell you why your mail was marked as [SPAM].
The X-Spam headers are what we are interested in.  Below are some
example headers of what I am talking about:

X-Spam-Status: Yes, hits=6.2 required=6.0 tests=AWL,BAYES_99,
DATE_IN_FUTURE_96_XX,HTML_70_80,HTML_FONTCOLOR_BLUE,HTML_MESSAGE
autolearn=no version=2.60-rc5
X-Spam-Report: * 0.1 HTML_FONTCOLOR_BLUE BODY: HTML font color is blue
* 0.1 HTML_MESSAGE BODY: HTML included in message * 5.4 BAYES_99 BODY:
Bayesian spam probability is 99 to 100% * [score: 0.9979] * 0.1
HTML_70_80 BODY: Message is 70% to 80% HTML * 2.6 DATE_IN_FUTURE_96_XX
Date: is 96 hours or more after Received: date * -2.1 AWL AWL:
Auto-whitelist adjustment

In the case of the above mail, the main problems are that the mail was
mainly HTML, and the date was more than 96 hours in the future (date
on senders computer was wrong).

Basically give us the entire headers and you will have an exact
answer.