Yahoo^® mail options

Pay particular attention to the three items indicated by the Red arrows.
The 1st causes the full header to be displayed on opened emails. This allows you to check the senders details and if you know what to look for can give an indication that the source has been spoofed. Also if you take the trouble to report an email as spam you should do so with the full header showing.
The 2nd prevents the possibility of a web beacon from alerting the spammer automatically that your email address is both valid and active.
The 3rd prevents you from automatically opening the next email.
It is possible that the next email has malicious content which is activated when it is opened, remember not all spam arrives in bulk mail or is stopped by junk mail filtering.