This really depends on your engineer.  The common formats are all digital, but if you’re a guy who records to 2-in tape, there’re engineers out there who work with it.  The general rule is, though, you want to give your engineer the highest-possible quality mxidown you can provide, in whatever format the engineer will accept.  So don’t send a maxed-out MP3 file.  Usually a 24bit, 44.1khz audio file in a standard, uncompressed format (wav, aiff, SDII) is sufficient.  Again, discuss this with your engineer, as each may have different requirements