Make sure "Use system proxy settings" is not selected in Thunderbird (this could happen for new setups), and if you need to set up a manual proxy, to leave "Use this server for all protocols" UNchecked.

For the live.com account, use outlook.office365.com for the POP or IMAP incoming server on port 995 (POP) or 993 (IMAP), SSL/TLS security, normal password authentication, User name = your live address, For the outgoing server, smtp.office365.com on port 587, STARTTLS security, normal password authentication, User name = your live address,

http://kb.mozillazine.org/Hotmail

For gmail, imap.gmail.com on port 993 and smtp.gmail.com on port 465, both with SSL/TLS security, OAuth2 authentication and User name = your gmail address. Cookies for google.com must be allowed.

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

Tonnes said

Make sure "Use system proxy settings" is not selected

I looked in my advanced connection settings and found out it was in fact set to "Use system proxy settings." I'm not using a proxy, so I set that to "No proxy." Unfortunately it didn't fix the problem. I also tried the "Auto-detect" option, which had no luck either.

sfhowes said

For the live.com account, use outlook.office365.com for the POP or IMAP incoming server on port 995 (POP) or 993 (IMAP), SSL/TLS security, normal password authentication, User name = your live address, For the outgoing server, smtp.office365.com on port 587, STARTTLS security, normal password authentication, User name = your live address, http://kb.mozillazine.org/Hotmail For gmail, imap.gmail.com on port 993 and smtp.gmail.com on port 465, both with SSL/TLS security, OAuth2 authentication and User name = your gmail address. Cookies for google.com must be allowed. http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

I tried those settings after setting the "No proxy" connection, but it still doesn't seem to be connecting. With the live.com address, it gives me the error message if I try to "re-test." For gmail.com, the "re-test" button seems to work, but then when I click "Done" it gets stuck on "Connecting to imap.gmail.com..."

I realize you're on Ubuntu, but do you have any security/firewall software that could be interfering with the account setup or connection?

sfhowes said

do you have any security/firewall software that could be interfering with the account setup or connection?

I don't have UFW enabled and I haven't installed any other security/firewall software. It seems possible there is something on my company's network getting between my computer and the internet. Ubuntu and Thunderbird are not officially supported, but they do allow both. They provide a guide for the mailbox configuration settings in Thunderbird, which gave me the same kind of problems I got with gmail and outlook.

Besides the factors you mention, the only other pitfall I can think of is it may be necessary to allow access to 'less-secure apps' on the gmail and live websites. This is separate from the issue of app passwords, which are only necessary if you impose 2-step verification, which you already said you had not.

I won’t say it’s impossible, but all accounts failing in a similar way due to required app passwords would be a small coincidence.

Try removing system-wide proxy settings for FTP and SOCKS by updating /etc/apt/apt.config if they are present, or find/add them if they reside elsewhere in Ubuntu if the file is not present.

Acquire::http::proxy "http://<username>:<password>@<proxy>:<port>/";
Acquire::https::proxy "https://<username>:<password>@<proxy>:<port>/";

Some report choosing Manual config and disabling OAuth2 should help (which may be related to app passwords), and I think, changing the method back to OAuth2 afterwards when they have been set up.

(Source)

When helpful, please let us know whether you think this is a bug in Ubuntu or Thunderbird.

Sorry for the delayed response. I tried adding the lines to /etc/apt/apt.config as suggested. The file was not present before, but adding it did not fix the problem.

It appears it is not a bug with Thunderbird, as I have tested the email client Geary on the same computer. It failed to connect to the same accounts. I suspect it is either a problem in some other Ubuntu configuration or my work network.