Below is a list of the most common SPF include mechanisms used by third party email service providers.
For each email service provider the DNS Lookup count is also listed.

Amazon SES
include:amazonses.com
DNS Lookups: 1

Google
include:_spf.google.com
DNS Lookups: 4

Mailchimp
include:servers.mcsv.net
DNS Lookups: 1

Mailgun
include:mailgun.org
DNS Lookups: 3

Mandrill
include:spf.mandrillapp.com
DNS Lookups: 1

Microsoft Office 365
include:spf.protection.outlook.com
DNS Lookups: 1

Mimecast
include:_netblocks.mimecast.com
DNS Lookups: 6

Postmark
include:spf.mtasv.net
DNS Lookups: 1

Salesforce
Include:_spf.salesforce.com
DNS Lookups: 2

Sendgrid
include:sendgrid.net
DNS Lookups: 1

Zendesk
include:mail.zendesk.com
DNS Lookups: 1

Zoho
include:zoho.eu
DNS Lookups: 2

Single include SPF record example:

v=spf1 include:amazonses.com ~all

The above SPF record indicates that only Amazon is authorized to send emails on behalf of your domain.

Multiple includes SPF record example:

v=spf1 include:mailgun.org include:_spf.google.com include:spf.protection.outlook.com ~all 

The above SPF record indicates that Mailgun, Google and Microsoft are authorised to send emails on behalf of your domain.

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