How to ask a professor via email about possible postdoc positions?
Generally such positions are advertised and often must be. Adverts are placed in such places as professional journals/newsletters and in newsgroups related to the field in question. Given this practice, blind letters may not even be answered and you can assume that there are no positions available. 

Alternatively, if your relationship to your advisor is good enough s/he can ask around among acquaintances about the possible availability or future availability of such jobs. The advisor probably won't be ignored. 

A third possibility is to attend a suitable conference or other meeting and get to know people and explore the possibilities in person, making yourself known, but also trying to build a network of future collaborators. 

Your letter is probably fine, but it will probably be trashed. There are too many blind appeals for people to spend any time on them. In the old days of departmental secretaries it was different, but no more.