I have 3 email accounts. When I send an email to a certain recipient from one address I get a delivery failure but works from another account. I actually have 3 recipients that I get a delivery failure message when I send them an email, all other addresses I send to work ok.
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Maybe these 3 recipients have blocked your email address ie. just the one which gives failure status.
Are you trying to send any email attachments? I had a similar problem a while ago and discovered that this was the problem. It’s just that some email servers have a small limit on how large incoming files can be so if you are sending an email with a large email attachment then they will just reject it and bounce the email back to you. If this is your problem then I can recommend for sending large email attachments. You can send them using your usual email program or from their website. They have free and paid accounts. They strip out the attachment and then send your original email with a link to the attached file to your recipient so they can download the file from their website. Hope you get to the bottom of the problem.