I suggest you get rid of your outgoing anti virus mail scanner and see if the message goes away. I most likely will.

Why do I say this? Because the messages about timeouts come directly from the server. The process took too long and it is not playing anymore.

What could make it so slow and result is an incorrect timeout? Software sitting between Thunderbird and the server.

What is the most common software sitting on an internet connection between you and the server. The anti virus program you trust to not be a bother and that usually is.

Hello Matt,

Thanks for your response. Prior to seeking Mozilla Support Forum help I left a support request with Kaspersky, my Internet security software source. They responded shortly after you did and suggested that I try pausing the Kaspersky protection, essentially the same advice you offered. (My own surfing for solutions had indicated that this issue seems to involve Kaspersky frequently, and that turning it off would reveal if this was the issue. I was reluctant to do that until both you and they suggested doing so.) I did, and sent a self-addressed email with nearly 6 MB of attachments and it processed without timing out. I'm awaiting their follow-up to my request for a more practical approach than disabling Kaspersky every time I want to send an email with more than minimal attachments.

Again, thanks for your help!

Starjourner

Puzzle this one.

Your anti virus product has scanned your system and certified you have no virus or malware that it can detect. This same product is scanning outgoing mail looking for the same thing it says you do not have. These things do not spontaneously arise!

So disable outgoing mail scanning. It is at best useless and at worst fraudulent method of stick advertising about themselves into your outgoing email.

The second part of this is anti virus products invariably scan the contents of the temp folder and every part of the email, has been in the Temp folder as part of the assembly of the email.

Now to the downside of temp folder scanning. Windows does a remarkably poor job managing files in this folder. There truly is a need to run the disk cleanup utility on the start menu / windows administration menu This will clean up what could be a million orphaned temp files that simply need to be deleted, among it's other functions. They do not harm, except slow down file access as the database grown over time because it is full of rubbish. One of the reasons windows slows down over time. Simply clearing the temp folder may resolve the issue, although you need to actually think about the security software you install and what it is doing. It is common for security suites to reduce performance more than 10%, often the higher rated the product the more it slows your system.

Matt,

Thanks for your assistance, and sorry for the delay in responding. In the interim since your 6/14/19 post I have been and remain in contact with Kaspersky about the issue. They have asked for information, including screenshots of the problem being encountered. I'm in the process of preparing a response.

That said, I have an additional piece of information for your consideration. Given the problem emailing with Thunderbird, I attempted to replicate the problem with an email sent using a Yahoo email account. It worked fine. Same attachments as the test message mentioned in my original post from 6/13/19.

If Kaspersky is scanning outgoing email, would it not be doing so for both Thunderbird and Yahoo? If so, and the Yahoo email gets through fine, it points to the problem being with Thunderbird. Or some peculiarity in the manner in which Kaspersky interacts with Thunderbird that does not arise when Kaspersky interacts with Yahoo email.

I'll mention this to Kaspersky as part of what I provide to them. What are your thoughts regarding this added information? Thanks.

Starjourner

Matt, et al.

Good news to report. I've been working with Kaspersky and the problem appears to have been solved. Their analysis from information they requested identified two apps, Avast Secure Line and YTD Downloader, as being incompatible with the Kaspersky Total Security app I use. After uninstalling the Avast and YTD apps, I was able send to myself and receive a test email containing 5.6 MB of attachments using Thunderbird. The email was properly recorded in Sent folder, and the attachments properly opened. So, congratulations to the Kaspersky Support Technicians for ridding me of this headache, which I hope is gone for good!

Starjourner