Scanning mail traffic for malware and spam is required even if you are using an external mail service such as gmail.com.
Vulnerabilities always exist, and because the software updates that close vulnerabilities are often installed too late, cybercriminals can exploit the same "holes" for years. In light of this, it’s impossible to be sure that the channel for transferring mail from your company to a remote service (and back) is absolutely reliable.
By exploiting a vulnerability, cybercriminals can try to intercept mail traffic and modify it.
You also cannot be sure that the anti-virus and anti-spam used on the remote service (if they are indeed installed there!) do a quality job filtering malware programs and spam, including the latest samples of them that haven't yet been analysed by anti-virus laboratories.
Doctor Web’s specialists recommend that you scan your company’s incoming email traffic. This can be done using Dr.Web Mail Security Suite products. For example, using Dr.Web for Unix mail servers, you can configure so-called transit traffic scanning.
The capabilities of Dr.Web for Unix mail servers allow it to be used in both companies that have their own mail servers and companies that do not have their own servers.