SendgridToolkit::AbstractSendgridClient#api_post calls HTTParty.post using the options key "query" rather than "body".
When using the key "query", HTTParty appends the urlencoded options to the URI.
With a large x-smtpapi header (eg. sending to hundreds of recipients) via Mail#send_mail, this causes SendGrid's servers to return a "414 Request-URI Too Large" (they also incorrectly return the error as text/html rather than just headers :/ ).
Since SendgridToolkit sends all requests as POST, I think it'd be safe to always send the request parameters as the request body rather than in the query string.