A business acquaintance emails me and always has [SPAM?] as first word in the email subject line. How can this be removed from the email subject?

The [SPAM?] prefix is most likely being added onto the subject line by your ISPs email server. Your email server detects that the emails are potentially spam (perhaps because the senders email address or email domain, being the portion of the address after the @ symbol) has been blacklisted as a potential source of spam. In this situation, there is no way you can remove the [SPAM?] from the subject line, since this is being done at the ISP level before the email even reaches your computer.

The second possibility is that the email is being tagged by spam software on your computer. If you have any antispam software installed on your computer, try temporarily disabling the software to check whether the problem persists. If not, then there should be an option in your spam filter to put your business acquaintance’s email address into a white list so it does not get tagged as spam. Without knowing the exact spam filtering software you use I cannot provide specific instructions, but if you could provide this information then I will attempt to provide further advice.