Try disabling your anti virus from scanning outgoing email. What anti virus do you use?

Hi Matt I am using Windows Defender, I will try to disable the anti virus from scanning my outgoing mail......thanks.

In the case of windows defender, it has no email scanner, try creating an exclusion for Thunderbird.exe. See https://support.microsoft.com/en-au/instantanswers/64495205-6ddb-4da1-8534-1aeaf64c0af8/add-an-exclusion-to-windows-defender-antivirus But include the path to the file in program files as discussed in the next link as not doing so creates a big security hole. https://www.reddit.com/r/Windows10/comments/5gf38v/when_excluding_a_process_in_windows_defender_do_i/