in mac mail is there a way to edit the subject line of an email thats in the inbox, this is just for internal filing reasons, say you get sent an email with the subject line 'attn:sam' that dosnt really help me if i want to go back and find it so could i rename that subject line some how ?
While technically it's possible, it's not trivial. You would have to write an applescript to search/replace the contents of your subject field and then use apple mail filters and run applescript based on your rule.
This is commonly done in the unix world via procmail, for example when spamassassin detects spam, procmail appends SPAM to the email subject message.
it might be better to set up rules based on subject to filter your mail into separate boxes for organization.
Actually there isn't a way for renaming the "Subject" field. It wouldn't be the right way in my opinion.
I think you should leverage on "Mailbox" and/ord "Smart Mailbox" for organizing your mails. They help you when you have to find a mail for a specific "topic".