SPF stands for Sender Policy Framework. In effect, it is the list of authorized senders for email delivered on behalf of your domain. Many customers will already have an SPF record created for their private domain. Domain's hosted under GSuite or Outlook365, for example, should already have an SPF record in accordance with the requirements of those networks.
Setting up SPF will require that your domain's DNS be edited. If you're not comfortble doing this, please consult with an IT specialist before making any changes.
The edit itself is simple; add the reachmail spf record to your own existing spf:
SPF is added as a TXT record to your top level domain. For the Host/Name you may need to leave this blank, use an @ symbol. See the your DNS provider's documentation for additional information.
A complete SPF entry might look like this:
"v=spf1 include:spf.reachmail.net include:aspmx.googlemail.com -all"
Your DNS provider likely has additional documentation on how to set up records of this kind using their user interface.
CAUTION: do not remove your existing SPF record. You should only add to your existing SPF record if one is already present. Removing an existing SPF record may negatively impact delivery of mail sent from previously authorized location used for personal mail or other mail streams.