user send and received hung

i have one user encounter sending same email 2 twice as the email stuck at outbox.

what she usually do is compose email first then then click send&receive to send out the email as she is from customer service dept.

she discover that some email are going out 2 times as she click send&recevied two times as she thought that the email is still at the outbox. The progress bar stuck there showing emails not send out completely.

could her mailbox be corrupted?