The phrase "unprofessional manner" refers to behaving or conducting oneself in a way that is not appropriate or acceptable in a professional or work-related setting. Full definition
If written in unprofessional manner the recruiting personnel are unlikely to waste their time in even glancing through it. (grandresume.com)
People who make public in a very unprofessional manner how they hated their place of work and try to discredit the company or employees of said company, at best will not be getting interviews to serious companies that perform background checks, and at worse the ex-employee may get sued for slander or defamation. (sbnation.com)
It's been an incredible eye - opener for me to see how many professionals regularly miss deadlines, are unresponsive to communication, produce poor quality work, deflect responsibility for mistakes, and otherwise act in a completely unprofessional manner. (smallbusinessesdoitbetter.com)