Basically queued email means that the email you are trying to send is not sending. Maybe you have attached any file with it. After the queued popup goes then the email is stored in gmail's outbox. Mostly this is caused by attachments. In other words; The Gmail Queued Email is the sign that the email is not sent from your end. Although it is stored in the Outbox, it is either waiting for the internet or sufficient internet speed to send it.