As the title points out, I don't want to say "I will send you an e-mail again" to somebody. Instead, I try to rephrase that sentence, so does "knock your door" address the same meaning in an e-mail conversation?
No, that does not have the same meaning.
In a professional or respectful setting, a good option would be
I will get back to you (next week).
The (next week) part is optional.
Just as a fun sidenote, if you're talking to very close friends in a very colloquial setting, you could say "I'll hit you up (later)." This is slang in the United States for texting/calling/communicating with someone later. This definitely shouldn't be used in any sort of setting with coworkers or even family.
No, that would not make sense.
I will knock (on) your door
does't really have any other meaning other than literally knocking on someone's door.
Knock usually takes a preposition.
I suppose you could use the slang
I'll come knocking
which means I will come look for you, but it implies physically going and looking, and it might be a stretch. However, if you still need a phrase that has a physical contact connotation, then :
I'll be in touch
is possible, and by email can always be added at the end :
I'll be in touch by email.
but that has the quality of being in contact in general, not specifically about emails.
Are you asking about sending an initial email, a followup email, or an additional email?
I find the again in your question ambiguous