How professional should my query for a PostDoc position be?

When asking about possible PostDoc positions what would be a good grade of professionalism? How long should the mail be and what should be included? (merely asking if somebody knows something would only take a line or two)

Obviously you want this to be professional (e.g., "Hey dude, I need a job" isn't great in any case), but not necessarily very long or formal. Your milage on this will vary based on the discipline, geographical region, and the individuals involved. However, in STEM fields and assuming this is a professor in the US or Europe, I would opt for an extremely short and informal mail to establish first contact.

Dear Prof. Farnsworth / (or: Dear Rupert, if you are on first-name basis)

as you may know, I plan to graduate in summer and am currently evaluating postdocs options. Given our joint interest in FTL travel, I was wondering whether there could be any openings in your lab that would fit my time line. Alternatively, are you aware of any postdoc stipends or grants that we could jointly apply for?

Thanks a lot in advance,

xLeitix

In my experience, such a super-informal mail has the highest chance of actually being read and is entirely sufficient to get the discussion going. Let the prof. decide what documents, if any, he actually wants to see before progressing (letters, CV, just a link to your Scholar page, etc...).

Important disclaimer: this really only works because the OP has stated that he assumes the potential postdoc advisor will recognize him when he receives the mail. If you have never met the person, or suspect that he won't be able to match your name to your face, add at least a link to your website. If you don't have a website, build one before doing anything else related to your job search.

If you know them on a “we recognize each other at conferences”-basis, I would recommend bringing the topic up at a conference. That is, if you plan to go to any conferences during this last year.

From personal experience, I can tell you that this works very well. In my case, my entry to the conversation was the fact that one professor introduced an upcoming project during his talk. After the talk I asked him pretty straight forward, whether he still needed postdocs for that. I also told him from the start that I did not know whether I really want to stay in academia and that I will postpone my career decisions until after my thesis was done, so I can focus on that first.

Neither my informal approach, nor my insecurity about future plans seemed to have a negative impact, as he told me immediatley to apply for his group at any time, if I am interested. And he renembered the talk three months later, when I was done and applied in a formal way, including CV, and all the other stuff.

To sum up, I would recommend an informal first approach (in person if possible), and a formal application where you refer to the last meeting, once you are certain.

Edit in response to the comment: If you really intend to get the position, if they have one, I would suggest a rather informal test in the email, but attaching a formal application that contains your motivation, some bragging about why the professor should consider you for the job, and the other usual stuff ;)

If you are just scouting and are not sure whether you really will apply, even if there is an open position, I would keep it informal for now. You have still a year left for a concrete application afterwards. (For me, the time span between the actual application and starting the postdoc position was about two months. They would have taken me already after one month, but I needed time for moving.)