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Here is the equivalent C# code for the provided Perl `check_email` subroutine:

```csharp
using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        // Example usage
        string email = "example@email.com";
        bool isValidEmail = CheckEmail(email);
        Console.WriteLine($"Is Valid Email: {isValidEmail}");
    }

    static bool CheckEmail(string email)
    {
        // Regular expression pattern for basic email validation
        string emailPattern = @"^.+@(\[?)[a-zA-Z0-9\-\.]+\.([a-zA-Z]{2,3}|[0-9]{1,3})(\]?)$";

        // If the email address contains invalid syntax or does not match the pattern
        if (Regex.IsMatch(email, @"(@.*@)|(\.\.)|(@\.)|(\.@)|(^\.)|" + emailPattern, RegexOptions.IgnoreCase))
        {
            return false;
        }
        else if (email.Equals("YOURCCMAILID@cibc.ca", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }
        else if (email.Equals("firstname.lastname@cibc.ca", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }
        else
        {
            // Email verification passed
            return true;
        }
    }
}
```

This C# code uses a regular expression pattern for basic email validation. The pattern checks for the required elements of an email address, such as characters before the '@' sign, a valid domain, and optional square brackets for IP addresses.

The conditions for checking specific email addresses (e.g., "YOURCCMAILID@cibc.ca" and "firstname.lastname@cibc.ca") have been adapted to C# syntax using `Equals` with `StringComparison.OrdinalIgnoreCase` to perform a case-insensitive comparison.

You can use this `CheckEmail` function in your C# program to verify the validity of email addresses based on the specified conditions. Adjust the conditions as needed for your specific requirements.