Q. Does CMOS weigh in on whether email subject lines should be capitalized in sentence style or headline style?
A. Great question! CMOS doesn’t cover email subject lines, but if we did we’d make a distinction between personal email messages, on the one hand, and formal announcements or mailings, on the other.
For personal messages (even if they’re related to work), sentence style would usually be appropriate. Not only will sentence style come across as less formal than headline style, but it will save the sender the trouble of determining which words get capitalized and which do not.
Subject: Last year’s Halloween party
Formal messages sent out by an organization, on the other hand, may be subject to the same editorial scrutiny that a press release or similar document might get. In that case, Chicago’s version of headline style would be appropriate for organizations that otherwise follow Chicago style (see paragraph 8.159 for the principles of headline-style capitalization).
Subject: Supporting Our Community through the Coronavirus Crisis
Note that not all styles apply headline-style capitalization in the same way. For example, whereas Chicago and MLA lowercase all prepositions in a title (including “through” in the example above), APA and AP capitalize all words of four letters or more. AP further distinguishes between titles mentioned in text and headlines for news stories, prescribing sentence style for the latter. So be sure to review your organization’s style before applying it to formal email subject lines.