Complex sentence: Modern approach is that e-mail sender must ensure that a server is a 8BITMIME one before sending any high-bit-set octets, and if it is not, then an appropriate MIME transfer encoding must be applied.
Split into simpler sentences: Modern approach is that e-mail sender must ensure that a server is a 8BITMIME one before sending any high-bit-set octets. If this is not the case, then an appropriate MIME transfer encoding must be applied.