How secure is a post doc job offer in an email from the department chair prior to receiving the official written letter? I'm in/from the US. I'm finishing my PhD this semester. I've been offered a research postdoc, via email, by the dept. chair at one of my top choices, but outside the US (would rather not give too many details). I replied to say I'm accepting the position. Everyone seems very happy: me, the chair, the professor I intend to work with. But I have nothing in writing.

In the meantime I have an offer for a teaching job at another place much closer, which I would definitely take if I were not getting the above postdoc. I don't feel comfortable turning this down until the other setup is official.

I emailed the postdoc place asking for an official letter. They said they would send it out by mail, but ignored my request for a scanned emailed copy, and I need to give a yes or no answer to the second place before their alleged letter would arrive. I emailed them again about the scanned copy and no response though it has only been a couple days.

How confident should I be that this postdoc is really happening? How common is it for informal agreements like that to be backed out of? Are scanned emailed letters less customary outside the US? It would be terrible of I turned down the teaching job and then the postdoc fell through.

Also, what should I say to the other place if I am still unsure about the postdoc by the deadline for my yes/no?

Update: I got the requested scan of the official letter today. I ran it by my adviser and he says this seals the deal and I can cancel any other interviews. Whew!!!
Don't count on an offer until you have it officially, in writing.  A verbal offer is not an offer.  An email or call saying you will get the offer is not an offer.

You say you "emailed the postdoc place".  Who did you email?  If it is an administrator they may not understand the urgency of the situation.  I would email the professor you will be working for and (politely) make it completely clear that you have a non-negotiable deadline for replying to another offer.  You should also state that if you don't have at least a scanned copy of an official offer from him/her before then, you will have no choice but to accept the teaching position, even though the postdoc is your first choice.  If they really want you, they will take the 5 minutes to scan a copy of the letter.  If they don't, maybe it's best to turn down that offer anyway.

In the meantime, I would also negotiate with the second place for an extension of the deadline.