Add Email to Salesforce allows you to push any emails you receive in gmail to your Salesforce account. We do the look up based on the sender of the email, so we can currently only use this button for emails you received - and not emails you sent.
We are working on changing the functionality to allow to log emails you have already sent, but in the meantime, if it didn't automatically log when sent, or show up in your failed activities in the activity log you can forward it to your Salesforce BCC email address.
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