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Immediately after trying to send the email, do you see any output in the error log by going to Configuration > System Info > Error Log?
In the campaign, can you please check whether you have set this email as Transactional or Marketing and whether it is set to Send immediately? When you select the campaign from the Campaigns list in Mautic, are there any pending or failed actions relating to this email?
I think it’s not likely a problem with your email gateway beause sending this email as an example works, but just in case, which email service have you integrated with Mautic?
This information should help the community get some clues to try and find a solution.