Email backup ISP IP not registered to the domain

Installed a backup internet service since our main line goes down occasionally.
When sending mail on the backup line the mail will get blocked since it is not a registered IP for the domain.
Is there a way to have both IPs registered for the same domain?

The issue is only when sending not receiving.
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Patrick BogersDatacenter platform engineer LindowsCommented:

Please look into adding a SPF record to your public domain, it tells the world which email server is allowed to send mail.

jplatovskyAuthor Commented:
So my SPF record would be    IP1    IP2

This would not affect the inbound routes?
Patrick BogersDatacenter platform engineer LindowsCommented:
Hi again,

SPF records live outside your internal domain so that doesnt affect anything.
Next you should check you excisting DNS records and create SPF record.

You can check with your DNS provider how an spf record looks with them, they can differ slightly.

Example could be  v=spf1 a include:publicipadress ?all

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