re :The mail server responded: 5.7.1 Spam detected by content scanner. Message rejected Please check message and try again."

That is a bounce back email message from the server and has nothing to do with Thunderbird. It is either the smtp server she uses to send or the server receiving the email. If you are both using Optimum then it is the spam controls used by Optimum.

The Message was scanned by the servers spam control program and it objected to something and therefore it rejected the message.

re: tried to reply to an email I sent her The bounce back error message from server suggests something is wrong with the content. Did the email contain any signature or do you both have signatures? Did it contain links ?

re :The tech asked me to sign into Optimum's online email and try sending a message. I was able to send messages without any problem. The Optimum tech said that the problem was with Thunderbird.

If your wife or you can logon to Optimim webmail account and send messages this has nothing to do with the issue as emails sent via email clients will be using smtp server. This only shows the tech support does not have much knowledge or does not care because the webmail works fine.

Good info here: https://www.optimum.net/pages/mailblocking.html

Your email address may have blocked. Your IP address may have been blocked. As you both have an issue from home, I would start by changing the IP address being used. The link above offers further info to check whether you are blacklisted.

Are you both sending using optimum outgoing server? Or sending from another eg: gmail but it is going to an optimum account?

Yes we both use optimum, I agree it is the common point for the problem. Her PC is Windows 7 mine is Windows 10 we are running different versions of Thunderbird and most importantly.. we did not change anything on our PCs when the problem started happening, our versions of Thunderbird were not changing.

My latest chat with Optimum they had me toggle this button called SPAMSCRUB but it did not work

We do not have Signature enabled for either of us... and there were no links in messages, we tried a very simple "HI THERE" message

The tech once again tried to tell me that if we can send with Optimum's online email then the problem is Thunderbird. I told him Thunderbird said that was not the case. Optimum online mail would not be using SMPT to Mail Server

Changing IP address will be a little hairy for me. I'm told I would have to do a factory reset of the router because it saves the IP. I haven't reconfigured a router for a while now.

Yes both my wife and I use Optimum Outgoing server

Toad-Hall, thanks for taking the time to try and help with this

You might ask you provider to actually use their own server logs to explain why exactly your mail is being flagged as SPAM by their SMTP SERVER. (I assume you are getting this in a dialog in Thunderbird and not as an email later)

When Thunderbird attempts to send mail it connects to the SMTP server and attempts to send the mail. If te server objects and offers up a message such as you have posted as a response, it is the providers server that s rejecting the mail. If you get an email later with that error and a server name, then it is the server named in the email message that is objecting.

In the first instance the connection, error and the reasons for it will be logged in the providers mail server and are accessible to appropriately authorized staff with your provider.

It is up to you to push your way past the phone answering script monkeys to the real technical people that understand the mail process and can check the logs and answer the question why did their server reject that particular mail. If you do get an answer, please post it. It is unusual for ISP's to actually answer real questions. They would much prefer you to accept that if you can use web mail they have fulfilled their obligation. They provide a service for a fee which includes the access to an SMTP server for sending mail. If it does not work, you may be entitled to a refund from them for whatever period it does not work as advertised.

So I've gotten NO WHERE with Optimum. They refuse to acknowledge the problem is theirs. I'm thinking I might just take their modem and exchange it for another to see if that clears it up.

Wyzard said

So I've gotten NO WHERE with Optimum. They refuse to acknowledge the problem is theirs. I'm thinking I might just take their modem and exchange it for another to see if that clears it up.

I doubt it will fix anything. But I have listed a few "try" options for you that might have a positive result.

Turning it off the modem for an hour or two and turning it on might get you a new IP address (unless you have a fixed IP) that in itself might change things if it is the ip address they are blocking. (I saw this in another thread here for another provider this past week where it worked.)

There is also a discussion here http://forums.mozillazine.org/viewtopic.php?t=574630

Whilst is has no relevance in spam filtering, I am seeing more and more bone headed spam filtering come out of US based ISP's so you might want to try changing the value showing in your mail.smtpserver.smtp*.hello_argument to Optimum and see if that has a positive result. Follow the instruction given in the discussion. Mine are just to brief. For more information on the config editor click the link.

Try sending a plain text email. Hold the shift key and click write. The compose window should be missing the normal formatting toolbar with fonts etc just type yourself a message, something simple other than hello. Include a subject and send it to an email address other than the one your sending from. having the same from and to address can raise the spam score of an email.

I tried the TEXT only email attempt but still the same error. I've checked 3rd party spam blockers used by Optimum and did not find our PC IPs on any list. I'm thinking of making Optimum exchange my cable modem to see if that might fix things.

I want to thank ALL that replied. It was VERY RIGHT of yous to suggest the problem was Optimum's and not Thunderbird. It took a lot of badgering via FaceBook to get Optimum to assign a "Back End" Engineer to get involved with the problem. Once they did they fixed it within a couple of hours. It turns out that somehow our IP was indeed flagged as SPAM. No idea how that happened since we DO NOT SEND SPAM! The Engineer said that if we send an email to someone and they mark it as spam, and if enough people do that we will be flagged as spam permanently. We don't do spam!!! We have a select few people we mail back and forth to with daily going-ons. All normal socializing

So Thank you all for you suggestions and knowledge

I don't know how to show that this problem is solved, and actually all your replies, in the end, solved the problem by insisting it was Optimum's problem all along