Have you found the 255-character limit for your SPF record to be too short? The way around this limit is to merge multiple records together using the "include" feature of the SPF ruleset:
The example below is of an existing record that needs to be modified for a list of IPs.
example.com.au. TXT "v=spf1 +a +mx +ip4:221.121.129.3 -all"Create a new txt record:
_spf1 TXT "v=spf1 +ip4:221.121.129.2 +ip4:221.121.134.34 include:m.google.com ~all"Once the new record is created, modify the original record and use:
example.com.au. TXT "v=spf1 +a +mx +ip4:221.121.129.3 include:_spf1.example.com.au -all"Once this DNS change has been made and propagated, your SPF records will now include both sets of rules.