I think you or your wife has made a typing mistake in the website name : it is not mail.optonline.net but mail.optionline.net - the second section (in uppercase) should be OPTIONLINE

According to their website:

incoming IMAP/POP server: mail.optimum.net port: 993/995 security: SSL/TLS authentication: normal password User Name: email address

outgoing SMTP server: mail.optimum.net port: 587 security: STARTTLS authentication: normal password User Name: email address

https://www.optimum.net/FAQ/#/answers/a_id/3897

According to the website http://www.mail.optionline.net/, all the URLs quoted by sfhowes containing OPTIMUM are its subsidiary pages. There is no valid URL http://mail.optonline.net/' - which is what was causing the problem - that URL must have been somehow changed by the questioner or his wife!

This may not solve the problem of getting email via Thunderbird, but is an indicator of where they need to go to correct their Thunderbird. settings.

I received the above settings that you indicate from Optimum, however I was unable to change them. I am using POP now. Have tried to change to POP3 and also IMAP. When I put in changes the "done" key does not become live. I can hit retest and will get the reply - "Thunderbird failed to find the settings for your email account." I tried setting it up as existing and also as new account, same result either way. My wife does not remember changing any settings, but.... Her email has had problems for a week, mine only started on Friday?

First, if you already have accounts set up, note that you can't change between POP and IMAP just by changing the server settings - you have to add the account as a new account with the correct settings. I recommend setting up accounts offline, from File/Offline/Work Offline, to avoid TB looking for the account settings. Start from File/New Existing Mail Account, enter your credentials, then click Manual config. and enter the settings. When you are finished, go back online.

I just tried offline. There is no button for done or continue, etc. Just cancel and advanced config. (see attached). If I hit advanced config I get "Error creating account Incoming server already exists". From this screen the menu is not available to return to online without losing the settings. Do I need to delete the existing account first?

If you already have accounts set up, give them distinct names in the Account Name field, at the top of Account Settings when you select the account in the left pane. TB applies the email address by default, and all accounts must have different names, even if they are on the same server.

I only have one account. I tried changing it's name with same result.

re :I only have one account....I am using POP now.

Let's check what you currently have got setup in thunderbird.

In Thunderbird Help > Troubleshooting Information Mail and News Accounts section

Copy all the data in that section and paste into this forum question. Do not alter anything.

OK so this shows I have several accounts? When I opened the account settings it only showed one. It was in a pane titled "account name".

Help/Troubleshooting, click about:profiles, create a new profile, and add the account, choosing POP or IMAP with the settings in my first reply, entered through Manual config. Once the account is verified, you can transfer address books, local mail etc. from the old profile to the new one. This will be easier than unraveling all the failed account settings in the old profile.

I was able to eliminate the extra accounts. I attempted to create a new profile, but could not find it using the directions above. (I am on an iMac running v 10.12.6) I was able to get on to Firefox and create a new profile there (hopefully that's the right place). When you say "add the account" I am not sure what you mean. There was nothing on the page indicating add. It was created and was below the original which is labeled as Default. Last night Default had the message "This is the profile in use and it cannot be deleted." Today that message is attached to the new profile. Also, when I log out I now have a new icon on the desktop - other user... I am still unable to enter the information for the settings. I hit the retest button and get the message Thunderbird failed to find the settings for your email account. Not sure what to do next? Thanks.

Thanks for all the help. I had a friend help me by following many of the solutions that had been posted. The problem appeared to be solved and then returned. He ran a malware program that discovered problems that kept changing my settings. All appears to be resolved now. Thanks.