What does 30 days notice in writing mean?

[Charles D.]

My wife just fired her property management because the owner has been playing games about charging her repairs on her house without giving her the price before the works and deducted 3 months of the rents plus she also have to pay over $7,000 in addition to  the works which only partially done and ignored even after several complaints. The contract requires 30 days notice in writing for the termination. She sent out a letter via the priority mail USPS on 24th of this month and the letter still in route to the property management office. She worried it will miss the time for the 30 days period until the end of March 31th. She emailed the owner of the property management at 10 pm on 2/28/20 about the termination of the contract. The owner email is his company email address and not his personal email address. My question are 1) does the email with her signature consider legal document? 2) The owner may say he did not read the email until he is at his office on Monday 3/2/2020, then can they charge my wife another month of management fee until the end of April? Thank you in advance for any helps.