SPF stands for Sender Policy Framework which prevents spammers from sending unauthorized emails from your domain. Spam filters setup on the recipient mail server, using SPF will sometimes prevent your emails from receiving in the users inbox.
Spamming is using an unauthorised domain is called spoofing. Sender Policy Framework (SPF) is an email security method to prevent spoofing from your domain.
To make sure you are sending from an authorised domain specially using G Suite setup SPE TXT record in your DNS zones. Follow these instructions from Google support.
-all means only this IP is authorized to send mail for the domain.
~all,softfail,Allow mail whether or not it matches the parameters in the record
+all,pass,Allow all mail
1. Sign in to your domain account at your domain host (not your Google Admin console).
2. Go to the page for updating your domain's DNS records.
This page might be called something like: DNS management, name server management, or advanced settings.
3. Find your TXT records and check if your domain has an existing SPF record.
The SPF record starts with "v=spf1".
If your domain already has an SPF record, remove it. If not, skip to step 4.
4. Create a TXT record with these values:
Name/Host/Alias—Enter @ or leave blank.
Other DNS records for your domain might indicate the correct entry.
Time to Live (TTL)—Enter 3600 or leave the default.
Value/Answer/Destination—Enter v=spf1 include:_spf.google.com ~all.
5. Save the record.
Check SPF Settings: https://mxtoolbox.com/spf.aspx