There are generally several reasons for the possibility. The background of the referee (such as a famous professor, etc.). ); There are some differences between the content and its own conditions, and so on; If you apply for a scholarship, then this must be checked.
As for your question, just remind the referee to check the email (in fact, it's okay to write the email yourself, because in most cases it's a phone call).
The most important thing is to write a letter of recommendation on school paper (with title, contact information, etc. ), and signed and sealed by the referee; It's best to use the school envelope, and remember to stamp it.
In fact, the real recommendation letter is sent by the recommender himself, but most applicants will send the recommendation letter together with the application form. Generally speaking, the seal here is a seal, which can be signed at the seal of the envelope or stamped, because in foreign countries, the referee will sign the seal of the envelope or seal it with wax. .