Prefix Setting for Email Subject

I am now struggling to set a prefix to email subject.
FreeToastHost has provided a prefix setting, however, it works only when return the received mail.

We use Japanese for a subject sometimes, but FreeToastHost does not accept the subject starting with Japanese. So, I tried to put a prefix of alphabets or figures, using the prefix setting option.
However, it does not work when firstly sending email. As a result, FreeToastHost does not accept and the mail is not delivered at all.
Once the mail is sent and when returning, the prefix setting works.

Is there any suggestion to set the prefix when creating a first mail?
 

I'm not sure that I understand your problem.  The prefix (documentation:  support.toastmastersclubs.org/doc/item/email-settings ) is automatically inserted when the email is sent. 
FreeToastHost only accepts UTF-8 characters in the subject - not just the first character 

Thank you for the reply.
Actually, emails with the subject starting with a few UTF-8 characters, even followed with non-UTF8 characters, are accepted.
Therefore, for an email with its subject starting with non-UTF8 characters, we expect that the prefix setting automatically set UTF-8 characters when email is sent for the first time.

The problem is that the prefix setting does not set when issued firstly, namely, sent for the first time. The setting does work after once email is successfully sent, then returned or  forwarded.  

Does this explain the problem?

Thank you for the reply.

Now, after the several Q&A, I realize that the email system need to verify the subject first, then follow the prefix assignment.
Therefore, the subject, firstly, should be UTF-8. Otherwise, the system rejects the email.

Thank you again for taking care of me patiently.