This will happen if you use user transaction when calling sp_send_dbmail. Do not call sp_send_dbmail with in user transaction.If you open a transaction and
send a mail with some attachment in first connection and dont commit . Open a second connection and send a mail using sp_send_dbmail it will not be queued and wait
till the first connection commits.You will also see preemptive_os_getprocaddress wait for second connection