There is no guarantee of anything in email marketing except, perhaps, that being dishonest in a Subject Line will lose subscribers. Is there anything you can do to increase your open rates?
One way of looking at a Subject Line is as a newspaper headline. They have, in essence, the same function: to get people to read on. Different editors prefer different types and it is almost axiomatic that you can tell which paper your are reading by the headline.
The variations are deliberate as they are meant to appeal to different sections of the public. Horses and courses.
You will have segmented your list and the criteria you have used will, to an extent define the Subject Line in very much the same way. So follow their example.
A sub-editor might feel that an article in the finance section of a broadsheet does not need to be as precise as one in a tabloid that does not have a picture of a bikini-clad woman above it. Whilst everyone says you should keep to 50 characters or fewer for an email, if your subscribers are busy and unlikely to open anything they do not know is relevant to them, then use as many words as you need.
If you segmented your email list on interests, such as funky sounds, then jargon might well be useful. See IT magazines.
Shocking headlines can work, such as ‘Double-decker Bus Found on the Moon’. Although it fooled only the foolish, it also increased sales of the [now defunct] Daily Star. Why not ‘Old Style Email Marketing is Dead’ and then explain why standing still is going backwards?
Received wisdom is that you should avoid jargon in headlines. However, in specialist supplements we see all sorts of phrases which are understandable only by aficionados. If you are trying to sell something to specialists then ‘epicyclic trunnion’ might assure them it is of interest.
It is difficult to get that little edge over your competitors. One way it to pinch the ideas of others. If you go outside of email marketing for methods, such as newspapers, you could well come up with something new. But no double-decker buses please.