Bill Ward wrote:

I used to be able to look at tracking headers in earlier versions of
Outlook but have for some reason not been able in 2010.

The PocketKnife Peek add-on for Outlook helps to look at the headers
along with showing the text and HTML versions of an e-mail.

I tried in outlook first but did not know how to change it at the
server.

Were you able to log into her Gmail account using their webmail client
in a web browser? If so, those are the same login credentials you use
in Outlook, too.

It said that the Outlook password would have initially been
issued by our ISP but I put it in as it is a word that is personal to
her as are the other outlook ones.

What is "it"? Outlook has no clue nor cares about who created the login
credentials for an account. Your ISP has nothing to do with Gmail.
Your ISP never assigned any password for her Gmail account -- unless
your ISP contracts with Gmail for their e-mail service. Some companies
and schools contract with Yahoo to provide e-mail services. Maybe Gmail
does, too. However, from your description, you have an e-mail account
with your ISP which is not a Gmail account, so your ISP and Gmail are
separate service providers.

VanguardLH wrote:

https://www.google.com/search?q=gmail%20insecure%20app

OK.

Does "OK" mean you check the server-side settings in your account and
found "allow insecure apps" was enabled? Or are you just acknowledging
the reference to the article.

Are you using POP or IMAP to access your Gmail account?

pop.

Did you configure your anti-virus software to intercept your e-mail
traffic to inspect for malicious content (e.g., attachments)? If the
AV's proxy goes dead or or misconfigured, you cannot reach the mail
servers from your local client through the AV's proxy to reach the mail
servers.

If your AV scans your e-mail traffic, disable it in the AV and retest.
E-mail scanning is superfluous, anyway. It afford no more malware
detection coverage than does its on-demand (real-time) scanner does
(because the e-mail scanner uses the on-demand scanner).

What did the entire error message say in Outlook?

Send test E-Mail message. Your E-Mail server rejected your login. Verify
your user name and password for this account in account settings.

And you had no problem logging in using their webmail client?

When you use the webmail client to look at your Gmail account's
settings, is it configured to allow POP or IMAP or both? You could
enable both both only need the protocol access enabled that you chose to
use in your local e-mail client.

It is set for POP.

So you did get in using the webmail client. Are the login credentials
you used in their webmail client the same ones you used in Outlook?

Can you see newly received e-mails when you use their webmail client?
When you send a new e-mail to the Gmail account, can you see it arrive
in Gmail's webclient?

Can you send new outbound e-mails when using their webmail client?

At the bottom left of the list/preview pane in Gmail's webclient is your
current usage of your account's disk quota. Lots of users are hoarders
and keep every e-mail they've ever received. What does the "xxx (xx%)
of xxx GB used" message say?

When logged into Gmail, go to your account and look at device activity.
Are those devices all yours?