There is a trade off between decentralization & reliability. Traditional E-Mail that people are talking about is NOT TRULY decentral.
For instance think about how E-Mail work:
1- Mail Client Query MX record from DNS
2- Connect to SMTP Server and Exchange E-Mail.
3- SMTP figure out the sender IP and look domain of E-Mail
3- SMTP server query SRV Record of that domain and check is that IP valid.
4- (Nowadays) SMTP query DKIM (Which is TXT record) and validate also.
1- The massage ONLY can access in TWO DAYS!
2- All Nodes have ALL E-Mail in Cryptic Form.
About you last sentence:
It's better that your message Not arrive instead of being disclose! and This is the philosophy behind True P2P IM systems.
Regards