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First you have to go and add SMTP Mail Server configuration in your Jira Server. It should look like following, add your gmail address in From address fieldsand Email prefix.
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After that if your Test connection is not working, follow these steps further.
Important: To create an app password, you need 2-Step Verification on your Google Account.
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Go to your Email configuration in Jira and now use and copy generated 16-character password in password like below.
Now if you make test connection it should be successful !
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Now you should be able to send an email to gmail account using for example Power Action application or any other apps for example that are sending emails insid inside Jira.
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These are proper settings for SMTP Mail Server configuration :
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