Yo answer your question, I am importing from a gmail account and all mails are in 1 single mbox file.
From what you wrote I am starting to think I used the wrong separator.
There is another from separator:
From 1621016496672657672@xxx Thu Dec 27 14:43:19 +0000 2018
But this one does not have out company mail: tikalk.com rather “xxx” so I thought I need to use the one with out company mail in it.
I will try with the following regex as separator regex: “^From .+@xxx.+”
I will update on my progress.