Does SQL Server Database Mail use threads while sending mail?

We are using SQL Server 2008 Database Mail to send emails to our website visitors. I don't know whether SQL Server sends mail one by one by picking up from the queue or it will use threads to send emails in simultaneous fashion.

If Database Mail uses threads, is there any way to increase number of threads to run concurrently?

Taken from Books Online: Database Mail

To minimize the impact on SQL Server, the component that delivers e-mail runs outside of SQL Server, in a separate process. SQL Server will continue to queue e-mail messages even if the external process stops or fails. The queued messages will be sent once the outside process or SMTP server comes online.

Database Mail use Service Broker technology:

Database Mail provides background, or asynchronous, delivery. When you call sp_send_dbmail to send a message, Database Mail adds a request to a Service Broker queue. The stored procedure returns immediately. The external e-mail component receives the request and delivers the e-mail.

The actual email delivery is handled by your SMTP server and so this will bear the brunt of the workload and should be provisioned according to capacity/email traffic requirements.

See Planning for Database Mail