Hello Chen -
The limit of 30 open requests applies to ACOM, not to the individual e-mail accounts.
Normally the requests are quickly purged when the data packet is complete and ready for your download. When there are communication errors, the packets become stranded (Request# unknown) and therefore not purged.
I have just cleared out some of the stranded packets. You may try your CAM-Chem download request again.
Carl