Complex sentence: A wide variety of character are technically valid for the local part, but mailsystem often limit their users choice of name to a simpler subset, and may in some cases also may make it difficult or impossible to send to perfectly valid external addresses.
Split into simpler sentences: A wide variety of character are technically valid for the local part, but mailsystem often limit their users choice of name to a simpler subset. In some cases they may make it difficult or impossible to send to perfectly valid external addresses.