A Sussex MP has met the county's chief constable to ask why he was investigated after he sent an email to a constituent containing swear words.
Conservative MP Tim Loughton said being questioned by detectives was humiliating and upsetting.
Sussex Police said an allegation of malicious communication was made and was fully investigated.
The force passed a file to the Crown Prosecution Service (CPS) which advised no further action should be taken.
The CPS said serious allegations had been made and the matter was thoroughly reviewed.
In the email to the constituent, the MP for Worthing East and Shoreham referred to the man's "whingeing self-serving poisonous bollocks that seems to have become your hallmark".
He said: "It's not language I use to any other constituents but I have to say that I, local councillors [and] council officials have been subject to quite a lot of abusive behaviour over many years.
"I felt that the only appropriate way in this case, having investigated many claims before made by this constituent, that the only appropriate way was to use his own language which I took from his own communications."
After the meeting, Mr Loughton said: "I came away with basically a load of old pap from the chief constable saying that he was perfectly happy with the way this whole thing had been handled."
He said he had been left seriously worried about systemic problems in how Sussex Police was run and would be meeting the police commissioner and raising the issue in the Commons.
Sussex Police said chief constable Martin Richards regularly met MPs and Monday's meeting was set up on this basis but it later emerged Mr Loughton wished to discuss his individual case.
"In usual circumstances, the chief constable would not involve himself personally with cases of this nature, as this would not be appropriate," a spokesman added.
But he said in the light of Mr Loughton's very public airing of his concerns, Mr Richards would be confirming to the MP the force's investigation of the allegations was carried out in the same way as it would have been with any ordinary member of the public.