I wouldn't feel comfortable having your login credentials. Can you try with port 587 and enabling TLS?

Same error 550 Mailbox unavailable

Are you sending to an actual real user at your company?

According to Yahoo small business (link to docs), the correct setup is SSL only - port 465.

Doc also says app password (link to docs) is needed (and issue says it was created too).

Other things I found were:

• Yahoo have had issues when you try and send emails from an alias. Check that it's not your case.
• Using the reply-to field helps on getting you properly identified (question in StackOverflow) and fulfilling server rules (it might be checking that you are saying you are sending the email from an address belonging to the domain)

I would try these two tests:

1. put into reply-to field the full address of the account you are sending the email from. Test the plugin configuration
2. Using TLS (no SSL) and port 587 (or empty, it'll use that port by default). Test the plugin config again.

Hi Andres,

Same error facing still with your information... too..., please find the screenshot. with gmail configuration still iam able send email from this plugin,... email sending success on gmail only....

Used email address is not an alias account

Any Update @alfabravoteam slide_o_mix ....

Iam Waiting for your response

I don't know about Andres, but I am a volunteer developing and managing this plugin. I have a day job that takes priority. You are welcome to look into the source and see if you can find the problem and submit a PR, or you can patiently wait until I have some time to look at it. Based on the error, the mail server is returning an error saying the mailbox you are trying to send to is not available, or possibly that the mailbox you are trying to send from is incorrect. I do not believe this is an issue with the plugin itself.

Just as Alex, I'm just another volunteer using my spare time to try and help building this software. This includes trying to help others (like you) to use it. But my day job will always have a higher priority.

The fact other users report a normal behavior while you also can see how it works with other SMTP servers point out to issues with the service you're trying to use (yahoo for SMB). I use this plugin in my day job with office365 and gmail servers with no problem (as you can see yourself with gmail). And security implementations are rather standard, so there's no special settings for Yahoo.

Finally, the fact it showed the same error using TLS means this is not related to plugin implementation but to your mailbox settings/provider.

Have you tried those settings with any other client (e.g. Outlook desktop)? If you're unable to use those credentials in other places, I'd suggest you to reach yahoo support.