Keeping in mind that SPF has a hard limit of 10 recursive DNS resolutions, it would be nice if Symantec only use one DNS lookup in its SPF record instead of three (spf.messagelabs.com, nets1.spf.messagelabs.com, nets2.spf.messagelabs.com). At this moment, 30% of SPF DNS lookups (3/10) are used by Symantec.
This can be done by splitting Symantec SPF DNS records into clusters. For example, in our use case, we only have cluster5 (located at EU).