First, you have to go and must add the SMTP Mail Server configuration in to your Jira Server. It should look like the following, : add your gmail Gmail address in the From address and 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 step Verification on your Google Account.
If you use 2-Step - Verification and get a "password incorrect" error when you sign in, you can try to use an app password.
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Go to your Google Account.
Select Security.
Under "Signing in to Google," select 2-Step Verification.
At the bottom of the page, select App passwords.
Enter a name that helps you remember where you’ll to use the app password.
Select Generate.
To enter the app password, follow the instructions on your screen. The app password is the 16-character code that generates is generated on your device.
Select Done.
Create use app passwords and create a unique 16-digit pass that you will copy and paste to your outgoing mail in data Centre settings :
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Go to your Email configuration in Jira and now use and copy the generated 16-character password in the password like below.
Now if you make a test connection, it should be successful!
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Now you should be able to send an email to gmail a Gmail account using, for example, the Power Action application or any other apps for example that are sending send emails inside Jira.
These are the proper settings for SMTP Mail Server configuration :
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