Core email service reports that email message cannot be sent

Questions
1

After installing a package (SEO toolkit) and recompiling the site to live i get an error when emailing from forms, which now doesn’t send emails… :frowning:

“Core email service reports that email message cannot be sent.”

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I also get it on localhost, but there is not much explanation, and i cant really find anything online…

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This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/114504-core-email-service-reports-that-email-message-cannot-be-sent