LibreOffice cannot correctly print envelopes with the correct orientation using the Brother MFC-8710DW. The Insert Envelope wizard appears to allow one to choose the correct orientation (both envelope and text), but no matter what, the print comes out perpendicular to the envelope. The MFC-8710DW requires that A10 envelopes be fed the long way from end-to-end (=), as opposed to from top-to-bottom(||). The Insert Envelope wizard indicates that this can be done, but no matter what, the text is printed as if the envelope was inserted from top-to-bottom (i.e., perpendicular to the envelope).
please retest with up-to-date LibO release (currently available are 4.2.6.2 and 4.3.2.2) and give a look at Bug 77277 which is still about envelopes and a Brother printer.
I set status to NEEDINFO still waiting for reported feedback
it looks it's a duplicate of this other one. *** This bug has been marked as a duplicate of bug 82370 ***