We always send you a document / guideline with delivery specs for mix & mastering. Just a small summary: we prefer 24 bit files. The most commonly used file format is WAV or AIFF, 24 bits or 16 bits. Please export your mixes in the same original sample rate and bit-depth as your project. In case you work in 44.1 kHz, 48 kHz, or 96 kHz export or bounce your mix down in the same sample rate. MP3 will NOT suitable for mastering. We always send you a document / guideline with delivery specs for mix & mastering.