Very strange deal; does not happen with all emails, only certain recipients, and I find no parameters which are common to all that would create a point to begin figuring this out.
The problem- When I send reply or new email messages to certain recipients, my messages are never delivered. Absolutely no question the addresses used are correct and valid. I’m not getting error messages, no mail lingering in Outbox, and all show as being sent successfully. Same recipient can send email to me, which is received fine however whether I Reply or enter their address manually I am unable to successfully get email to deliver on their end.
Way beyond there; not something that commonplace. Also not being blocked by their hosts or domains. Even saved as approved contacts on several - no difference. And not receiving any NDR notices.
Problem sounds like the receiving mailboxes or receiving ISP.
If the emails don’t bounce back and it’s not in the spam / junk folder at the other end as @Gary advised , then the receiving mailboxes might have eg: filters setup to auto delete your email or that ISP might be blocking your actual ISP mailboxes.
First thing have the recipients add you to their whitelist mailbox filter and try sending again. If they still don’t get your email, contact the recipient via alt email address or phone and ask them to contact their ISP and advise you cannot email them and your emails dont bounce back.
Already confirmed to be on approved contacts and white lists. A couple of these recipients are running unfiltered and ISP’s have reported no issues. If it were only that simple.
Maybe contact your email provider. If it seems eM Client is uploading the messages, but they are not going anywhere, then they may be able to determine where the problem is.