"The link will expire after August 19"

This gives people a sense of urgency, but doesn't discourage them from trying the link if they're a little late. "Through" is (to my ears) an Americanism.

"valid through August 19"

I've always found "through" to be confusing when used like this. It's like the whole "Next Friday" thing - is that the one about to happen (it's the "next" one to occur), or is it the one after that? (in Australia, it's usually the later) I much prefer "valid until August 19"

(I'm a native English speaker, not from the US where "through" seems more common)

Right. "Through" is better than "until" because "until" leaves the reader unsure whether it includes August 19th. And 2011 is unnecessary; if someone does find an old message, they can look at the Date: header.

Personally, I would say "August 19th", not "19".

Agreed, especially with the dropped year.

If the duration is less than three months, the year can be inferred from the date of the email.