What are your settings? They should be:

mail.optimum.net on 993 (IMAP) or 995 (POP) and 465 (SMTP); SSL/TLS, authentication = normal password, User Name = part of address before the @.

https://faq.optimum.net/node/7327

What is the antivirus and its mail settings, and do you use a VPN?

I have tried every suggested setting and combinations of settings. Nothing seems to allow Thunderbird to recognize my account/password.

I solved my problem through shear perseverance and probably dumb luck. I just kept searching different phrases describing the email situation and came upon a "faq" answer sheet from Optimum. It explains the settings one should use for email. The settings I used are:

POP (Inbound) Incoming mail server: mail.optimum.net Incoming mail port (SSL): 995 (Secure Connection. Recommended if your mail client supports) Incoming mail port: 110 (Not recommended. Legacy mail clients only)

SMTP (Outbound) Outgoing mail server: mail.optimum.net Outgoing mail port (SSL): 465 (Secure Connection. Recommended if your mail client supports) Outgoing mail port: Port: 587 (Not recommended. Legacy mail clients only) Check My SMTP server requires authentication

                                                                        (My Note: This last check thingy is not on settings page)

This configuration fixed the problem. The Optimum faq info is here: link text I hope this helps others.

Hello jducky!

Thank you!!! You have solved the OPTIMUM/Thunderbird email issue! I followed your link and found the instructions on Optimum. My email is finally working now.

Thanks again for sharing the resolution... and for your perseverance.

viggy1121

Hi, viggy1121.

I am so glad I was able to help. I should call Optimum IT again and give them the solution! Perseverance definitely pays off. Happy emails!

jducky

What happens when @optonline.net address is replaced with @optimum.net address? Does your incoming mail addressed to @optonline.net still arrive or returned to sender due to incorrect address? Is it going to be our responsibility to inform all our mail recipients of the address change? Yesterday my chat with Optimum tech support said engineers are still working to fix the problem with Thunderbird. My problem started 2 weeks ago.

When was @optonline.net replaced with @optimum.net? I'm not aware of the change. I have had no changes in the address.

What happens when @optonline.net address is replaced with @optimum.net address? Does your incoming mail addressed to @optonline.net still arrive or returned to sender due to incorrect address? Is it going to be our responsibility to inform all our mail recipients of the address change? Yesterday my chat with Optimum tech support said engineers are still working to fix the problem with Thunderbird. My problem started 2 weeks ago.

An optonline email address will probably still work even if the server name is changed to one with optimum.net. Similar to how sbcglobal, pacbell, swbell accounts still work on att.net servers.