Mieszyslaw Malecki on Macedonian language 1938
...However, it should be added that in addition to the Macedonian paculiarities which go along the developing line of either the Bulgarian language or Serbo-Croation, there are also completely specific feature which do not appear in either of these two languages. Among the more important phonetic features of this category is the developement of *b, *b > Q, e; *a i.e, o, even kj, gj < *tj, dj, which are undoubtedly very close to the Serbian Ch and Dj, but still different from them.
Therefore, to the question whether the Macedonian dialects are Serbian or Bulgarian, I would answer that they are neither exclusively Serbian or Bulgarian, but rather the majority of them represent and individual type of dialect (which could also be called a Macedonian language) related closely to the two mentioned languages.
The Macedonian language is between Serbian and Bulgarian, and its inclusion in only one of these languages is, from the linguistic point of view, unfounded.
M. Malecki, Z zagadnien dialaktologii macedonskiej, Rocznik Slawistyczny (Krakow), XIV (1938), p.142