Why some of my mails do not reach their correspondent although seen as sent mails?

[Nukte]

I noticed surprisingly that some of my sent mails actually did not reach to their correspondents. They are seen as sent mails in my sent box. When I realized that my correpondents actually did not receive them, (because they called on phone when they did not get my mails as they were expecting them) I forwarded those mails, writting "did you receive this one" in the subject section. They recieved the second time. I am talking about two different correspondents. They have no filter in place, I am not blacklisted on their part and my mails are not in their spam holder.
I had the same problem a few times earlier before but did not take it seriously, I thought my mails mistakenly erased or smt. by my correspondent. But with the latest two incidents I think it is a serious problem for me. This problem happens both from my computer and Samsung mobile app.

[trooper6387]

it seems like you have all the bases covered. I have not seen this glitch before.
You might consider using some sites listed below so you know the status. Below that I will give you some contact info for Yahoo....Make sure you explain exactly as you did in this question.
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Yahoo does not have the option of notifying you when your message is received.. these do

Enter READNOTIFY.COM in your web browser and bring up their website....(Trial period is for two weeks or 25 emails) Then, you assign your email address to it and send your friend the email as normal only you add readnotify.com at the end, your friend won't see this. So for example, friend@yahoo.com would become friend@yahoo.com.readnotify.com
Then when she receives it, you will know when, how long it was opened for, if it was re-opened and if it was forwarded.

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You can these also.
http://www.getnotify.com/ (free)
http://www.spypig.com/ (free)
http://msgtag.com/home/ (free)
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and this one (free trial period-7 days)

http://www.pointofmail.com/
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In addition, you can enter EMAIL RECEIPT NOTIFICATION in your web browser to bring up other sites providing this service
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OUTLOOK EXPRESS....
When you create a new one or reply/forward an email you
can click on TOOLS at the top. Click on REQUEST READ
RECEIPT

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This link provides a phone number/email/ help center options
http://io.help.yahoo.com/contact/index?y…
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You can now contact Customer Care on Twitter
Once you start and tweet your questions you can access them
and see if they have been answered by clicking on "ME" when
you are on the site.
https://twitter.com/YahooCare
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And you can contact them on Facebook.
https://www.facebook.com/YahooCustomerCare

[aron]

hey Nukte, I am having the exact same issues. Have you managed to find an answer? If so, please share!