Job offer asking me to mail my diploma

In a nutshell, I am American currently living in America. I was offered a job (no interview or anything) in Japan. After a bit research, it is a reputable school and everything seems legit.

The school manager (or recruiter) is asking me to mail them the original copy of my university diploma and TESOL. Mailing my originals to Japan is just not something I'm willing to risk. I was offered the opportunity to work under a tourist visa while it processes if I can't mail them, but isn't that illegal?

I offered the school a copy of my official transcripts, but they're still unsure. What do you think?

Thanks.

The school manager (or recruiter) is asking me to mail them the original copy of my university diploma and TESOL.

This is common practice. You will get back the original afterwards. Alternatively, I understand that an original letter (not a copy) from the university that confirms graduation could also possibly be acceptable.

I was offered the opportunity to work under a tourist visa while it processes if I can't mail them, but isn't that illegal?

Yes, that is illegal, but probably not too uncommonly done when the employment schedule does not smoothly match with immigration matters.

I offered the school a copy of my official transcripts, but they're still unsure. What do you think?

The immigration bureau doesn't need the transcripts. They need the diploma.

Thanks! I'll be sure to inquire about a letter.