Cock.li has been through so much shit, after they opened registrations again.
Maybe they should've kept it closed...

Anyways, here are some alternatives.

>Mail.tm
This is my personal favorite temp-mail provider.
They often rotate domains, and their site/api is really nice.
While they are indeed "temporary mails", they actually come with a password, so you can get access to the account long after you created it.
https://mail.tm/en/

>Pissmail.com
This would be the best 1:1 Cock.li alternative.
Not as much a focus on temporary mails, but more the same shitposting stance to mail.
I don't trust them as much as Vincent just yet though.
https://pissmail.com

>Derpymail.org
If you're a brony, this email was suggest when Cock.li was on "red alert".
They have been in business for quite a while, and their community seems healthy, so probably a decent alternative to Cock.li.
https://derpymail.org

And below are just some free email providers, who doesn't serve as a direct alternative to Cock.li, but might interest as well.

https://disroot.org
https://mailfence.com
https://mailum.com

And some temp mail sites.

https://temp-mail.org/
https://smailpro.com/temporary-email
https://yopmail.com/en/
https://www.mailforspam.com/
https://www.guerrillamail.com/#
https://www.emailondeck.com/
https://mailnesia.com/