SPF records large than 256 bytes are pretty common. See e. g. aspmx.sailthru.com (1st what found right now). You just have to split text into multiple strings, but don't forget to add a space after or before a derective, because strings are joined without an extra spaces: https://tools.ietf.org/html/rfc7208#section-3.3 May be there are buggy implementations which don't understand this, but doubt they are common.