"automated email messages" near the top -- I would make this 'site-related email messages'
"There have been reports of "spam" emails" -- this makes it sound like we haven't been able to confirm the reports, like we're not sure if they exist or not. I would make that sentence:
"Lastly, there are unfortunately a variety of "spam" emails that reference LiveJournal accounts, but these emails are not authorized or sent by LiveJournal itself."
Tweaked wording: "Emails that are sent by LiveJournal itself are related in some way to a LiveJournal account registered to your email address." Saying 'your account' makes an assumption that they know they have an account, etc -- someone people get registered because of a typo, or an old account they forget, or whatever.
"You also cannot opt out of receiving security related messages, such as the automatic notification that your journal has been deleted; nor can you opt out of messages sent by LiveJournal staff or volunteers, such as those sent by the Abuse Prevention Team or responses to a Support request you opened." -- that's a really long sentence, and I'm not sure a semicolon followed by 'nor' is all that grammatically corret. I'd just split it to two sentences, starting the section with "Similarly, you cannot opt out of blah blah."
"You may need to use the Lost Information Page to find those usernames." The switch to 'usernames' there is confusing, should just stick to 'accounts'.
I would rearrange the first couple sentences in the 'not from LJ' section:
"LiveJournal does not send unsolicited commercial emails, nor is there a user-accessible email feature on the service which could be used to send such emails. If you are receiving messages that appear to be commercial spam, these emails are not coming from LiveJournal, even those that link to a LiveJournal account that may offer commercial products."
" Email "from" addresses " -- I would capitalize 'From' there in those usages. It's a field label, a title, etc.
"These accounts often contain objectionable content." -- I'd flesh that out a bit more. "These accounts often contain standard spam content, such as pharmaceutical or adult-themed ads."