How should we send account credentials back and forth by email?

When sending account credentials (ex: a Magento backend login or a Mandrill account login), I usually recommend using something like readthenburn.com which gives you a unique link that you can share with someone which will auto expire immediately after they open it.

One of the most common ways for accounts to be compromised is when an attacker gains access to email and then goes through email to find other account credentials.

And even if someone doesn't gain access to your email account, unencrypted email is fundamentally not very secure or private.