Both kanji has a similar meaning that mean to send or transmit.
But I wonder which and when the kanji should be used.
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
Basically, 送信 is used with intangible objects (signal, data, information, etc.), whereas 送付 is used with tangible objects (paper letters, books, goods, products, etc.).
When you send something electronically as emails or downloadable files, 送信 and 送付 are both used. Whether to use 送付 depends on how strongly the speaker feels it is a "thing" — perhaps IT professionals tend to prefer 送信, and elderly conservative businesspeople tend to prefer 送付.