One of our hosting customers recently had a lot of trouble sending emails.
After much investigation they found that because they were using Yahoo with Verizon some changes were required.
I am posting below his email to us and hopefully this will help someone else who finds themselves with the same problem.
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Yesterday I determined the solution - and if I were you, I would keep this info close by for your customers who use Yahoo! as their email provider. What I learned is the Yahoo! requires you to "verify" any email "accounts" that pass thru their servers. What the user needs to do is to go to the ISP webmail and then to OPTIONS, then More Options, to get to the email preferences page - then ACCOUNTS. You then need to ADD an account and follow the directions - which will then send to that account (mine was the ISP main email account [email protected]) and it was asking to verify [email protected]. I have attached a copy of the email I received.
I spent 2 hours on with Verizon Tech support and they could not figure out a solution. It wasn't until I started to look at Yahoo! (they told me that my email was "pushed through" Yahoo!) that I found a possible solution, which it did turn out to be. Verify the 3rd party email first.
In Outlook you need to have the accounts SMTP (outgoing) setup as the ISP requires (outgoing.yahoo.verizon.net for mine) and in the More Setting, Outgoing Server you need to select to Log In to your ISP email address.
I hope that this saves you and your other Yahoo! linked customers in the future. Mark this one as Solved!!
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I hope this helps!