what is the actual rejection error? My guess is it will be something like your reply to address is different to your sending address, but the exact wording would be very helpful.

Matt's response to my problem does not solve my problem as I see it. The following error information comes back when I try to email any email address at optonline.net; it must be noted that I have changed the email name address@optionline.net to protect the actual email address:

   It must be noted that I previously was able to email all optonline.net email addresses without problems.  When I email those optonline.net email addresses using my Charter.net webmail the email goes through and gets delivered without a problem.  So I feel it is something in Thunderbird 38.4.0 that is causing the emailing rejections as SPAM email.

This Message was undeliverable due to the following reason:

Your message was not delivered because the destination computer refused to accept it (the error message is reproduced below). This type of error is usually due to a mis-configured account or mail delivery system on the destination computer; however, it could be caused by your message since some mail systems refuse messages with invalid header information, or if they are too large.

Your message was rejected by mx2.optonline.net for the following reason:

    5.7.1 Spam detected by content scanner.  Message rejected. 

The following recipients did not receive this message:

    <lXXXXXXX@optonline.net>
    <tXXXXX@optonline.net>
    <dXXXXX@optonline.net>

Please reply to <Postmaster@msg.strl.va.charter.net> if you feel this message to be in error.

The following attachments have been removed from the bounce message:

Reporting-MTA: dns; mtain005.msg.strl.va.charter.net Arrival-Date: Tue, 22 Dec 2015 17:39:58 -0600 Received-From-MTA: dns; impout004 (68.114.189.19)

Final-Recipient: RFC822; <lXXXXXXX@optonline.net> Action: failed Status: 5.3.0 Remote-MTA: dns; mx2.optonline.net (167.206.4.79) Diagnostic-Code: smtp; 554 5.7.1 Spam detected by content scanner. Message rejected.

Final-Recipient: RFC822; <tXXXX@optonline.net> Action: failed Status: 5.3.0 Remote-MTA: dns; mx2.optonline.net (167.206.4.79) Diagnostic-Code: smtp; 554 5.7.1 Spam detected by content scanner. Message rejected.

Final-Recipient: RFC822; <dXXXXX@optonline.net> Action: failed Status: 5.3.0 Remote-MTA: dns; mx2.optonline.net (167.206.4.79) Diagnostic-Code: smtp; 554 5.7.1 Spam detected by content scanner. Message rejected. _________________________________________________________________________

    Thank you for your time and effort in resolving this about on-going email problem.

Your message was rejected by mx2.optonline.net for the following reason: 5.7.1 Spam detected by content scanner. Message rejected. Please reply to <Postmaster@msg.strl.va.charter.net> if you feel this message to be in error.

The message tells you that it was rejected as spam . So, I presume you followed the advise, told the person how identical emails are sent but are only accepted if sent via webmail. What did the postmaster say about it especially as it would seem only emails to one server?

It may be that the server only accepts certain email addresses - some kind of restriction they have imposed. Perhaps, it does not accept your IP address. Webmail uses different server, IP address etc, so the two are not the same.

but since it is rejected as spam - look at the Subject, the content and anything that is additional. Assuming the subject and content are the same as the webmail sent message, I would ask you to check the following: Do you have an image in your signature ? Do you use Anti-Virus scanning outgoing mail and does it add it's own information to the end of your email ?

Well, I finally found the answer to why my email messages have been sent back as SPAM. I finally called Optimum.net Technical Support and the problem was resolved in about 10 minutes. I sent an email to the email support and they said that the SPAM control switch was set on rejecting email being sent to the Optinline.net members. Once they turned off the SPAM control the email messages were sent through. Thanks to all of you who helped in resolving this issue, much appreciated.