See…

https://www.virustotal.com/en/url/921eee33f6814705f03fcfbda71c72ccf29100f22e2c878d217d66ba6eceee0e/analysis/
http://quttera.com/detailed_report/www.sharelatex.com
https://sitecheck.sucuri.net/results/www.sharelatex.com
http://online.us.drweb.com/result/?url=http://sharelatex.com/
http://www.dnsinspect.com/sharelatex.com/1437671686
http://dnscheck.pingdom.com/?domain=sharelatex.com+&timestamp=1437672185&view

It appears to be “not” your domain but the IP address that is blacklisted.
Going by http://toolbar.netcraft.com/site_report/?url=sharelatex.com%2F+ I would strongly suggest you
set up a “sender policy framework” for your domain.

edit: I would suggest you follow the advice Pondus gave above.