Spam Issue on a single mailbox

Sorry im not sure where to go.
So im looking for a script of some sort.
I have been struggling since the 24th with my mailbox sending out spam and then you get a huge amount of bounce backs. The domain is already blacklisted due to the huge amounts of mails bieng sent.

So heres the history im with A2hosting i have never had any issues with mails.
I have a wordpress site running thats all up to date.
Contact form has a captcha. And this mailbox in question isnt even linked on there.
On the Ftp side i did find some wierd files with wierd time stamp & a file *.contactsend I Deleted those.
I did a scan with some plugins and i didnt detect any malware or malicious code.
I also added a maillog plugin but there are no mails running through the wordpress site.
So i cant see where the issue could be.
Mail password has been changed ten million times.
When i suspend the outgoing mail server all the bounce back stops and when i unsuspend the outgoing mail everything starts all over.
I have now suspended the outgoing mail and forward those mails off to another server and that seems to be ok.

PC has been scanned theres no malware the account isnt even on outlook.
But i need to still find out where this code is lying.
Can anyone suggest anything