send as - ndr half of the times

Hello,

Hopefully someone can give me some tips were to look for the following problem:

We use exchange 2010 SP2 (no rollups) with outlook 2010 (in cached mode).

We have some problems with users using "send as".

If someone sends to a list of people as someone else first they get an ndr that the message could not be delivered to the following recipients. We open the mail from the sent items and choose to resend the mail and all goes well.

What could be causing this first error while the resend then goes without any problems?

Thanks for the help.

Regards,

JB