I do not know what you refer to by "technically correct", nevertheless let me offer some points.
Neither of the sentences is strictly grammatical as both appear to be missing a subject. This may be just fine in an email subject where one may suspend grammar for the sake of terseness.
One might also concede that for the same reasons, 'but' has been omitted from the first sentence, that (fully fledged) would read "I will log in but not before 12pm." With or without 'but', that's what the first sentence appears to be saying. Among other things, you commit to logging in eventually.
In the second sentence however, you delimit the period during which you are not going to log in, but without committing to logging in at all. Even though the addressee may assume you will log in some time, it is not strictly speaking what you have said.
So (although I think we're nitpicking here a bit) the main technical problem with the two sentences is that they have different meanings. However, I think both are good enough to get the message through.