But it's not a Save action though, is it?
The feature you describe sounds very much like the conventional Favorite feature. The user marks an item of interest to have it easily accessible at a later point. The item is still accessible even if the user doesn't mark it, but then the user has to manually look it up again. This does not sound anything like a Save feature. A user leaving an item without saving it conventionally entails that the item is unrecoverable, it was not of interest so therefore I discard it, usually used in the context of self generated material.
If I'm writing an email and suddenly decide to wait with sending it to a later point, I would expect a feature symbolized with a save icon and the description Save for later, a star icon would feel really out of place and inaccurate here. However, if I receive an email for example, and the email is of great interest (something I want easy access to) I would expect a Favorite feature symbolized by a star. Not a Save icon that says Save for later. "-So what happens if I don't save it? Can I still access it?" Would be my thought process in that scenario.
The Star icon is widely spread and pretty much a convention for marking items of interest. Just have a look right next to your question. Based on how I interpret your question I would without a doubt recommend you with going with the star icon.