To prevent a compromised mailbox will cause unauthorized spamming, we would advise all the hosting & server owners to be vigilant of trojans and malware that may be injected into their hosting.
What you need to do:
Firstly, change the affected mailbox login password immediately to a more complicated password consisting of numbers; symbols and upper and lower case letters.
Secondly, scan those Computers that you use to connect to the mailbox via Microsoft Security Essentials and update their Windows Security to the latest release (for Windows users) as well as do the necessary anti-virus scanning.
Next, update all website apps and CMS (Content Management System – such as Joomla & WordPress) and 3rd party applications for the hosting immediately. Some trojans and malware can get into hosting through these loopholes.
If you ever wonder – Why do I get bounce backs of emails I never sent?
However, if you require any further assistance, feel free to contact us.