No issues. Remember, it's always DNS! Meaning, you control what the outside world sees as your mail server through the use of MX records in DNS. When another server is sending an email along that's addressed to [email protected], the outside DNS server will lookup your MX record in your root domain example.com and find whatever you put in there as the MX record, in this case smtp.example.com, then it will initiate a connection to whatever IP address you identify in DNS as smtp.exmaple.com. If you want webmail setup on mail.example.com for your users, then that's also fine.
As was mentioned already, depending on your setup, you could just have one name for both like mail.example.com. In the example of using Microsoft exchange, OWA/Webmail usually listens on port 443. SMTP traffic default listens on port 25 and sometimes port 587. So when a user opens a browser and punches in mail.exchange.com, it will default to use HTTPS over 443 and your server will answer that connection with the webmail website. If an outside email server wants to send someone in your domain an email, that server will reach out to mail.example.com over port 25. Since the connection is coming into your server on port 25, your email server knows that this is SMTP traffic and will accept and route the email to your users.
