You used the "Generate DKIM" button in the Messaging Gateway and noticed that the resulting string was very long and contained quotes. You want to know if this is a proper DNS record string.
Release : 10.8.0
The format of the TXT record string is as such due to string length limits. In order to remain within the record string length limit (255 characters), but include the full record which is much longer than the string length limit, the DKIM text record string is broken up into parts that are each less than the limit. Each part is enclosed with quotation marks and is considered a single string. The application using the DNS record result will concatenate the strings into the full record.
Here is a good reference from the ISC, who is the maker of the Bind DNS software: