Cox says you should use smtp.cox.net on port 587 with STARTTLS security or port 465 with SSL/TLS security, authentication = normal password and User name = part of your address before the @. Sending on smtp is different from sending through webmail, so success with the latter does not ensure success with the former.

What are your settings, and did you change your password?

Hi sfhowes Thanks for taking the time to reply, much appreciated.

All my settings are as you described. 465 with SSL/TLS, and, normal password, and username as the first part of each email address. I changed my password as a last attempt to fix this last night, thinking perhaps it had expired (though receiving emails is working fine). All to no avail.

COX is essentially saying "sending by webmail works fine, so the problem is not at our end". As far as I can tell, nothing changed at my end. No Windows updates Saturday afternoon, T'bird itself was updated back in June. What still makes me suspect COX is they "upgraded" my email service last week, which is usually doublespeak for "we wanted to get rid of something at our end".

I use these email addresses for a business that I run, and saving my "sent" emails is important to me. Using Webmail won't cut it, and I really don't want to use Outlook.

Re-reading your info - COX told me to use SSL/TLS security with either port. Both tried, both failed. I had not tried the STARTTLS with 587 combination - THAT WORKED !!!

Thanks VERY much !!!

For any .NET developers coming across this, it seems that Cox is forcing all email access to SSL starting on 25 Jan 2018. For programmatically sending emails, the following code worked for me:

Dim lSmtpClient = New SmtpClient("smtp.cox.net") lSmtpClient.Credentials = New System.Net.NetworkCredential("AccountUserNameBeforeThe@", "AccountPassword") lSmtpClient.Port = 587 lSmtpClient.EnableSsl = True

I have this problem as well. Cox supposedly filed an incident and was supposed to get back to me within 48 hours - 3 days and still no response!

I tried the STARTTLS with 587 port as noted sfhowes reply above, but that DID NOT work for me. I have NO problems sending emails with the new Cox configuration, but can't send emails! I do not get the server timeout, but a login failure. I have reset the password to no avail. I have closed and restarted Tbird each time trying something new. Like sfhowes, the Web interface does not work well for me.

I'd appreciate some help. Thanks!

DcnDon said

I have this problem as well. Cox supposedly filed an incident and was supposed to get back to me within 48 hours - 3 days and still no response! I tried the STARTTLS with 587 port as noted sfhowes reply above, but that DID NOT work for me. I have NO problems sending emails with the new Cox configuration, but can't send emails! I do not get the server timeout, but a login failure. I have reset the password to no avail. I have closed and restarted Tbird each time trying something new. Like sfhowes, the Web interface does not work well for me. I'd appreciate some help. Thanks!

Start Windows in safe mode and see if you can send mail. If it works, it means there is a security program interfering with the secure connection. See this post for an example:

https://support.mozilla.org/en-US/questions/1199185

Thanks, I did as you suggested and started Windows in safe mode. The results are the same. I cannot send email and the error message remains. It appears that there is a problem with the login password, but it is exactly the same as the one used to retrieve email and that works fine.