How to use

for i := 0 to mailboxList.Count - 1 do
begin
  mailboxName := TImapUtf7Encoder.Decode(mailboxList[i]);
end;

Implementation

class function TImapUtf7Encoder.Decode(const AValue: string): string;
var
  n, h, t, k: Integer;
begin
  Result := '';
  n := Length(AValue);
  h := 1;

  while (h <= n) do
  begin
    t := System.Pos('&', AValue, h);

    if (t <= 0) then
    begin
      t := n + 1;
    end;

    Result := Result + System.Copy(AValue, h, t - h);
    h := t + 1;
    if (h > n) then Break;

    t := System.Pos('-', AValue, h);
    if (t <= 0) then
    begin
      t := n + 1;
    end;

    k := t - h;
    if (k = 0) then
    begin
      Result := Result + '&';
    end else
    begin
      Result := Result + DecodeBase64Imap(System.Copy(AValue, h, k));
    end;
    h := t + 1;
  end;
end;

class function TImapUtf7Encoder.DecodeBase64Imap(const AValue: string): string;
var
  enc: TclEncoder;
  b: TBytes;
begin
  enc := TclEncoder.Create(nil);
  try
    enc.EncodeMethod := cmBase64;
    b := enc.DecodeBytes(StringReplace(AValue, ',', '/', [rfReplaceAll]), 'UTF-7');
    Result := TclTranslator.GetString(b, 'UTF-16BE');
  finally
    enc.Free();
  end;
end;