E-mail etiquette suggests that all upper case/caps should not be used in email as it gives the impression that you are shouting or at the very least indicating strong emphasis. Does this apply to the Subject: line?
Of course there are exceptions to every rule, however in general, caps are an indication of emphasis as desired by the Sender. If caps are used in the Subject: field that can be construed as “bringing attention to” by raising your tone.
Another concern with his approach is you could be misidentified as a spammer because most spammers don’t understand the nuances of case.
If you want to bring attention, it would probably be better to structure your Subject: field to include both caps and lower case such as:
Subject: FOLLOW UP: Previous E-mail Subject
Subject: REMINDER: Tomorrow’s Meeting
In comparison, one could assume you were yelling/shouting/adding strong emphasis if you used caps in the Subject: field in the following manner examples:
Subject: WHERE’S MY ANSWER?
Subject: DID YOU GET MY E-MAIL!!!!!
See? It’s all in the words you choose and how you use them that can determine if you are perceived as yelling or creating an aggressive tone. Add those extra exclamation points and there will be no doubt by the other side that you are in fact intentionally making a very strong point!
It is clear by the first examples that you can use caps to draw attention in the Subject: field without giving a negative perception even before your email is opened. Just make sure you do so to relay the intended level of concern and emphasis.