I don't understand this:

This doesn't happen with the account that is linked to its own folder, just to the one that is linked to a sub-folder of the inbox.

If it's in a subfolder, how can it have its own SMTP setting?

I have three accounts, not three identities on one account, set up. Account "A" stores in inbox on local folders. Account "B" stores in inbox on a seperate folder from local folders. Account "C" stores, via filtering, in a sub-folder to inbox on local folders. I have done this for about 5 years with no trouble. With the upgrade to 31, account "C" will not send replies from the selected outgoing server. Instead, it uses the outgoing server setting from account "A". Account "B" has not shown this change.

"If it's in a subfolder, how can it have its own SMTP setting?"

Multiple servers can be set up in the "Outgoing Server" section of the Account Settings window and selected in the "Outgoing Server" box of the "Default Identity" page of each account entry (below the check box for attaching a v-card). This selection is being ignored.

New Info:

The trouble account is one I use primarily to do business correspondence. My work ISP won't set up the server ports to be compatible with standards for spam blocking, so my isp won't send mail through it. I therefore set up a seperate account (called "C" above) that the work isp forwards to. I have the settings on this account to show my reply to address as my work account. Until the switch to 31, replies to forwarded messages used the correct settings. Now they use the settings for the account that existed when I started using Thundebird (account "A" above). I have just tested and found that replies to messages sent directly to account "C" use the selected server. So it seems that 31 parses the header differently from prior versions.

Try creating an identity for your work account in the account settings for "Is it C" that they end up in.

The identity allows you to use an SMTP server for the account to mail was originally sent to

"My work ISP won't set up the server ports to be compatible with standards for spam blocking, so my isp won't send mail through it."

I get from that that your home ISP is comcast and their insane blocking of port 25. (Which does nothing but increase their revenues BTW, business DSL does not suffer this problem) and your work ISP insists you use Port 25 which Comcast rightly claim is for Server to Server communications.

Please if you can, name names so we can actually offer you a best fit solution rather than a load of might fit suggestions.

Basically there should be a workaround for your port issue as well, but without having details it is difficult.

"I get from that that your home ISP is comcast and their insane blocking of port 25."

Verizon Yahoo, but same attitude. Mail at work is a .com hosted by compuweb.com.

Using identity with compuweb server seems to not work. Not entirely sure I set it up right.

I assume you have already been here, but I must go there again just in case.

Have you tried port 587 with SSL? or port 465? both with or without SSL.

These have actually been designated outgoing ports for mail clients for a decade or so. So trying them might get somewhere. The correct designation is 587 without SSL and 465 With. Unfortunately not everyone bothers with standards.

But that does not really address the issue of something that worked not working.

Can you post the headers of an email that does not work.... you can email me direct if public posting is not an option.

Have tried to post header to you via message, but can see no evidence that it sends. Don't want to flood you with posts.

Second try at this post:

Found expedient solution staring me in the face, delete and re-enter account "C" info. New entry stores in its own folder and replies to selected server.

Thanks for advice and suggestions.

Glad you got it working in a fashion, but I would still like to know why it was malfunctioning. You could have achieved that same result by Tools menu > account settings > server settings and select the advanced button and telling Thunderbird not to use the global inbox.

Please email me the header.... my email address in in my profile here