I keep seeing 'SPAM-LOW' and/or 'SPAM-MED' and/or 'SPAM-HIGH' in the subject line of my incoming email.
Where is this spam coming from?

Short answer:
The email server is grading your emails and inserting that text into the subject lines.

Long answer:
The anti-spam controls on your email server employ a combination of Spam Assasin, Bayesian Filtering, a Sender Framework Policy (SPF) check and finally Real-Time Blacklist (RBL) checks against the SpamCop and SpamHaus RBL's.  
(follow links to learn more about each filter)

This combination of filters will assign a point value (weight) for each email message.
If this weight is over 10, the email is rated: SPAM-LOW:
If this weight is over 20, the email is rated: SPAM-MED:
If this weight is over 30, the email is rated: SPAM-HIGH:

The filters assign the following weights when an email fails (tests positive):

Filter	Weight
Spam Assassin	0 to 30
Spam Assassin can not be adjusted.   to have Spam Assassin removed from your domain eMail our support team by Clicking Here the following filters can be adjusted at the domain level
Bayesian Filtering	10
SPF Fail - Senders IP is NOT valid for senders domain	30
SPF SoftFail - Senders IP is questionable for senders domain	10
SpamCop	10
SpamHaus	10

When an email message is suspected to be spam, the email server will prefix the subject line of the email with the rating:
i.e. SPAM-XXX:  <-- where XXX = LOW,MED or HIGH            

The email message is not altered in any other way.