I'm not an expert, but it seems you have a trojan or some other virus that's hacked into your mail server and you need to get it removed. Run antivirus software to identify and remove the virus and it should solve the problem. I can't really recommend any specific software, except that Malwarebytes has worked for me in the past where others have failed. Worst case scenario you might have to buy a disk to scan the pc to find the hidden issues...they are good at cloaking themselves now and the free software might not be advanced enough to identify and remove.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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