When LinkU attempts to send an e-mail to someone "acting" as if it is coming from you, for example, "email@example.com", the receiving server may deny it, because it doesn't know if its being spoofed, or spam, or if it's really you. As a result you may get an error, such as "This message fails to pass SPF checks for an SPF record".
If you are not hosting e-mail or DNS with LinkU, then you will need add an SPF record to your domain name.
You may already have SPF records, if so please contact whomever is hosting your DNS, such as your website provider, or domain provider to edit the records as needed.
Here is an example:
v=spf1 ip4:126.96.36.199/27 include:linkumail.com ~all
This SPF record gives our servers permission to send mail on your behalf. Once you have added the record, you can test it using a site like: https://www.dmarcanalyzer.com/spf/checker/ to verify the SPF records have been added correctly.
This should resolve your issue for passing the SPF check.