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My application started randomly experiencing deliverability issues when sending mail to Outlook and Hotmail addresses. I'm sending mail in java 8 through the mail API Transport.send(msg). This uses a dynamic range of IPs to send mail and I've found the at least 3 of them are blacklisted currently. I guess this could be a big reason for why I am experiencing this issue. Has anyone experienced this before and how have you been able to overcome this issue?

In the meantime I'm trying to explore other avenues like using a Gmail SMTP relay.

Dave M
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Kevin
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    Microsoft is infamous for accepting mail and then dropping it on the floor, so if you're merely blacklisted you're doing better than many. The reject message has instructions on what to do. – Michael Hampton Mar 04 '21 at 18:46
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    If you are using Google App Engine, this could be an isolated case and the best course of action is to open a [Public Issue Tracker](https://issuetracker.google.com/) and express with more detail the issue you are having. – Jan Hernandez Mar 04 '21 at 21:58

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