If a user is getting an error that includes a reference to SPF (Sender Policy Framework) when emailing from Infusion
Server refused mail at END OF DATA - 550 5.7.1 Message rejected (SPF)
then the required action is for whomever hosts the sender's domain / email to add the senders email address (eg mxa.onenet.co.nz) to their txt spf record.
Some More Information
If abc.co.nz has an SPF record of:
TXT v=spf1 include:spf.protection.outlook.com -all
This means that a receiving mailserver that checks SPF records will look up the domain abc.co.nz and check the SPF record for the Mail Server associated with it. If an SPF record does not match the Mailserver the email is relayed from, the message will be denied - this is a common tactic to prevent against email spoofing.
In this case an email is being relayed from mxa.onenet.co.nz, and the gaining Mail Server is rejecting it because it's checking SPF records, and mxa.onenet.co.nz is not listed.
To prevent this, you can ask whomever hosts abc.co.nz to add mxa.onenet.co.nz to their txt spf record.