I am trying to get the address of the sender in mbox formated mails in python. When I get the line that contains the sender, it looks like From: Mister X <misterx@domain>. I am able to retrieve the mail address with, for example, re.findall('<[a-zA-Z0-9\.]+@[a-zA-Z0-9\.]+>', str). I think that should be fine since email addresses, as far as I know, cannot contain any other characters. What I do not understand is why the expression <*@*>, which I expected to match any characters in the email address does not work at all. In fact, re.findall('<*@*>', 'From: Mister X <misterx@domain>')returns ['>'].
Here is my answer. 
why the expression <*@*>, which I expected to match any characters in the email address does not work at all.
Because you are using re module that evaluate your expression <*@*> as regular expression.  If you want to make your expression evaluated as wildcard, use fnmatch module. But fnmatch have only function that checks if string matches or not. So you can't get matches by using fnmatch. From your question, It seems you want to retrieve mail address, so you shouldn't use fnmatch module. Just use re module to get matches.
I think you are just confusing regular expression between wildcard.
Matching address in mbox format in python