How secure are your mails? Not much.
Normally your mail travel using the SMTP protocol (Simple Mail Transfer Protocol). Note how the S in SMTP stands for Simple, not Secure.
Only few email providers are connected to each other using encrypted channels.
GMail is known for supporting encrypted communication with other servers, but only if also the other servers support it (obviously).
So, what happens when you send a mail from a GMail account to a Yahoo one? Everything is fine until the mail leaves the GMail server to travel to the Yahoo server: at this point the email is on a non-encrypted channel and can be captured by anyone listening on that channel, without warrant and without cooperation from the email providers.
This happens for most of the mails sent between different email providers.