To authorise Infradox to send emails on your behalf of your domain, add their mechanism to your SPF record. The mechanism for Infradox is:
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​ip4:217.149.135.209

Detailed steps can be found below.

DKIM is not supported directly on Infradox however Infradox supports SMTP relaying via any external SMTP server. 

Relaying via any server that supports DKIM (Gmail for example) will allow you to DKIM sign your mail.
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To test your configuration for both SPF and DKIM is correctly configured, start your 14 day free trial with OnDMARC!