He will be in the office on May 10th.

As far as the grammar is concerned, it means that the person isn't available until 11th May (or first working day thereafter), but I think it is often used to mean that the person is available on 10th May.
To avoid misunderstanding, you could say: "I'm out of office and can't be contacted / reached until 10th May"

What I've written is of course, now I come to look at it, not very different from the original.
"I'm out of office and can be contacted again from 10th May."

Damit alle es richtig verstehen, schreib vorsichtshalber:

I am on holiday/on a business trip and back in the office on 10 May (oder 11 May, wenn einschließlich 10. Mai gemeint ist).

Support PA; be very specific.

I think this may be a BE/AE understanding issue. "Until" in business AE does not generally mean "including"; for that, we use "through." So being gone until May 10 means that May 10 is the first day on which the person will be back. The OP's "10th of May" makes me think the source is BE, though, in which case "until" may well include that date.

Vielen Dank für die Antworten!

Offensichtlich ist es tatsächlich missverständlich.
Wobei in diesem Fall ("out of office until 10th of may") die Person am 10. wohl wieder da sein wird.

Werde wahrscheinlich selbst in Zukunft anhand eurer Vorschläge eine Formulierung wie folgt verwenden:
"I am currently out of the office and will be back on 10 May."

In Ordnung so?

Perfect solution. Go for it!
Couldn't be better if it had been from me.

sorry, just one little thing.... maybe an american thing, but if you are writing that for people in the uk you'll have to say 10th May....what I've just seen - ten May - makes the hair on the back of my neck stand up....

@nat: War mir selbst unsicher über diese Tatsache. Hab daraufhin die korrekte Schreibweise (für BE) gegooglet und gelesen, dass man in der Schriftsprache auch in UK das "th" weglässt (?).

Any further comments on this?

Das 'th' ist optional und, wie Kommas und Bindestriche, verschwindet immer mehr. Es geht also beides.

Oxford Style Manual recommends "10 May" and says to avoid "10th May". I think that when this came up before the consensus was that "10th May" is slightly more old-fashioned BE.

@8: I agree with #10 and 11 - 10th May is increasingly regarded as rather old-fashioned.

@8: I agree with #10 and 11 - 10th May is increasingly regarded as rather old-fashioned.