What I'm saying about MUAs and mail standards isn't a sweeping statement, it's just the way the system was designed, and a paraphrase of the published RFCs. I'm sorry if it was off-putting. The original designers of these protocols put in lots of requirements for retrying in the case of temporary failures, or notification back up the mail chain in case of permanent failures, etc., trying to make sure that nothing just disappears without a trace. My mail server (Courier-MTA) is obsessively standards compliant ;) and over the years I've been using it I've come to rely completely on its logs. If they say something was done, or not done, then I have never had any reason to doubt them.

T-bird is a GREAT program, and I'm sure that every effort is made to insure that it's totally standards compliant. The really useful and relevant piece of information in your post is that "in some special circumstances mail can be lost during transmission" when using it to send in background mode. I should note that I had the same problem using Intuit's resending service, which works via an email interface, probably SMTP, so it looks as if, in some circumstances, Windows interprocess communication has issues. No surprises there!

My guess is that, as you say, it's a problem with QB, or Windows. The first thing I did when I had the problem of lost emails was to disable outbound virus checking, but it made no difference. Mail still got lost until, on your suggestion, I put T-bird offline wen posting emails for re-sending from QuickBooks. ALL the outbound invoices and statements got saved in T-bird's Outbox, and T-bird had no problem sending them on when I put it online.

I might also note that up until last week I was using a QuickBooks 2011 on an XP system, using MS Outlook for resending invoices and statements and never had any problems. Forms sent from QB showed up in Otlook's Outbox queue when it was started. For some reason, possibly a MTU issue in the connection with the mail server, large emails stopped going out altogether and my bookkeeper migrated me QB 2014 on Windows 7 on a different system. QB 2014 offers T-bird rather than Outlook as a local mail user agent option, so this was a totally new ballgame.

Thanks so much for your suggestions and observations, Matt, which have helped a lot and hopefully saved us a huge amount of work going forward.

One thing to add, I have no personal experience but understand that send mail from 64 bit applications via Thunderbird simply does not work. Something about MAPI being depreciated and the switch between 32 and 64 bit modules. See the graphics here. As Thunderbird is not 64bit there is no 64bit stub to link to.

This sounds reasonable, and could explain the situation. I know that the Win 7 platform here about which I'm talking is 64-bit. I don't know about QuickBooks 2014, nor do I know how to determine its bitness on a Windows platform. I know that it will install on a Windows XP system, or so my bookkeeper tells me, most installations of which are 32 bit. The installer may determine the version installed.