The best way to whitelist our sender IPs is using our SPF record of spf.envoke.com. Read more.
You should also set up DKIM for your sending domain.
If you already added the SPF and DKIM records and need to to specify IP addresses, you can use the IPs below. Note that some of these are IP ranges not single addresses and they are subject to change without notice.
220.127.116.11/26 (18.104.22.168 - 22.214.171.124)
126.96.36.199/27 (188.8.131.52 - 184.108.40.206)