There is no definitive answer for this without knowing what ladybird uses for email.
if it's a online service, such as hotmail then the account password has been got in some way by a spammer, either by simply trying 10,000's until they get the right one, or by the details being entered on one of those sites that claims to tell you who has you blocked on hotmail (or similar) such as this fake one (cheers squarebear ;))
http://www.square-bear.co.uk/mitsuku/msn.htm
If it's a offline mail client on the local computer (such as outlook) then the computer has been infected by spyware, it's worth noting that spyware is different to a virus and not all anti-virus software will detect/prevent spyware (and some that say they so are not very good at tit)
If it's the former, simply change your account password, if it's the latter download and run a full scan with antimalwarebytes
http://download.cnet....-8022_4-10804572.html