MailChannels sends from many IP addresses and the list of IP addresses we send from changes occasionally. Fortunately, the list is always available in our SPF records and you can look up the list easily with online tools. We recommend using the SPF record lookup available at MXToolbox:
http://mxtoolbox.com/SuperTool.aspx?action=spf%3arelay.mailchannels.net&run=toolpage#
Dmarcian provides another useful tool for listing SPF records:
https://dmarcian.com/spf-survey/relay.mailchannels.net
As of October 5, 2018, the following single SPF record can be used instead of including the above domains if you require a “flattened” version to reduce DNS lookups:
v=spf1 a mx include:relay.mailchannels.net ?all