Hello @DebbyC98 ,
Welcome emails will trigger for newly added contacts that sign up through a sign up form. Welcome emails do not trigger off of other list additions - such as you uploading a file of contacts or manually moving them into such lists. Additionally, if you have an automation series that's set to trigger off of additions to lists covered by welcome emails, the automation series will override the welcome email and send its first step instead.
If you're wanting a welcome-type email to send automatically for contacts added to your lists from any source - you manually adding them, them signing themselves up, an integration sync, etc. - then I'd advise setting up an automation series with a list-join trigger. You can have just one step in the automation series, if all you're needing is that initial welcome.
See also:
Overview of Automation
Automation Tips
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