Spam sent from my mailbox to genuine users with same message and different subjects.

Spam sent from my mailbox to genuine users with same message and different subjects.

Hi,

I am finding that several of my clients receiving mails replied from my mailbox with same message as below. Such mails are sent are found in Trash folder in mailbox. Please look into my mailbox trash folder to see these mails sent and let me know why this is happening. The contents of mails is below :


You have pending incoming docs shared with you via Dropbox
 
Click to open:  Secure Message

Dropbox makes it easy to create, store and share online documents, spreadsheets and presentations.