Email system
You could build a mail system on your own server. None email goes through third party transfer, as to fully guarantee your infomation safety.
Send Email
OnceDoc email system uses Self-hosting server in default, with no third-party mail server. All OnceDoc workflow are bound with wechat, which could inform the relevant person through email and wechat.
Email Sending Interface
Sending an email is not a technical problem. Major mainstream mail vendors decide wheter to reject your email based on your IP, SPF settings and domain name, instead of your email contents. Please value your brand reputation, don't send spam at will to stay away from the blacklist of mainstream vendors.
Receive Email
Once you set your MX record in the domain name, you could receive emails through OnceDoc.
Since OnceDoc won't block any email in default, so as the Major mainstream vendors, it is unlikely not to receive your emails, whereas there may be some time delay with differert vendors.