Which to use depends on the context. The others appear to have interpreted your question to refer to people being in the meeting, which is correct. Given what you said about emails, I assumed you are referring to things that happened at the meeting. In that case, at is usually the correct option. I'm sure that you tabled your schedule at the meeting. We discussed that at the meeting. You were present at the meeting. But, if you are referring to 'presence' - then you use 'in'. Your secretary rang while you were in the meeting. If you can give an example, we can look at it and see if it is correct, and perhaps explain why.

When I read "at the meeting", I think of the person being outside the room or near where the meeting will be. When I see "in the meeting", the person is inside the room and is attending the meeting. When you meet with people in a room, it is more common to use "in". That signifies that the person is busy meeting with someone else or with others.

Toru, I just read Gary's post and realised that he is right. Perhaps you can provide us with a few more sentences of the email so that we can understand the context. Thank you for pointing that out.

Hi Toru, It should be: Someone is "in the meeting". The preposition "in" confers many meanings. The most common purpose is to indicate a position. Example: The stationery is in the drawer (position within a space). In your sentence, "in" is used to indicate an involvement in something. You are involved in the meeting. Other examples: He was in politics from 1991 to 1996. (He was involved in politics.) They were actively involved in volunteerism. (They were involved in volunteerism.) The managers were in the midst of a heated argument. (They were involved in an argument.) Toru is in doubt about the use of "in". (Toru is involved in some uncertainty about the use of "in".) Hope this helps. Cheers, Lance