If you have control of your mail server, then you should be able to tweak some options to get it to convert 8-bit messages or send them anyway, but what to do depends on the mail server.

If you do not (and this may be a better idea anyway), you should be able to change your mail client to use quoted-printable for email messages. This will convert non-ASCII characters to an ASCII sequence so that no 8-bit characters are sent. Again, how to do that depends on your mail client, but you should be able to find it in some preferences somewhere.