Never say " I beg to acknowledge " or " I beg to remain ", or use any phrase of that sort with beg in it. Beg in such a position can have no meaning. Its common use in this way is presumably to be accounted for by a false analogy with the reasonable use of I beg as a polite introduction to a contradiction, " I beg to differ ", or " I beg your leave to differ ". There is no reason why one should apologise, however faintly, for acknowledging a letter or remaining an obedient servant. See Commercialese.