Checks if a string is a valid email address in Python. Uses a regular expression to check that the email address conforms to the syntax rules specified in RFC 5322.

is_valid_email.py

def is_valid_email(
    email_str: str
) -> bool:

    """
    Check if a string is a valid email address. The regular expression checks 
    that the email address conforms to the syntax rules specified in RFC 5322.

    Parameters:
        email_str (str): The string to check. 

    Returns:
        bool: True if the string is a valid email address, False otherwise.
        
    Example:
        >>> is_valid_email("user@example.com")
        True
        >>> is_valid_email("user@example")
        False
    """

    # Use email.utils.parseaddr to split the email address into parts
    # and get the email address part.
    email_address = email.utils.parseaddr(email_str)[1]

    # Use a regular expression to check that the email address is valid. 
    regex = r"(?=.{1,64}@.{1,255}$)(?=.{1,64}\.{1,255}$)[^@ ]+@[^@ ]+\.[^@ ]{2,}$"
    return re.match(regex, email_address) is not None