Interesting dataset. Data like this can be used to identify strong links between contractors and government officials.
One problem is that the metadata should have only contained anonymized entries for the email addresses of the counterparties of the Seattle.gov addresses, the article leaves this unclear.
Another potential problem is that if a case of corruption or nepotism is identified that has not been passed to the authorities for review that the author suddenly finds himself in the possession of data that can be used to blackmail some fairly powerful people, in fact there might be fish at a higher than city level government in the trawl because there have to be links between Seattle officials and state officials.
Yet another problem is that the addresses most likely contain the names of private individuals (including employees) as well, and I am not quite sure what to think of that but feel that the city has no business releasing that in cleartext.
A better way for amateur sleuths and the city government to work together to battle corruption would be to release only anonymized data to protect the identities of the people working for the city, for instance by releasing only hashes of the email addresses, for instance a hash@hash format where the hash for all Seatle domains is released to the requester. All the relevant analysis could still be done, and if something interesting was found it could be released to law enforcement who in turn should have then used a judge to order de-anonymization of those entries they are interested in.