Is http://security/ubuntu.com down?

[Jim]

Hadn't used my notebook running Mint 18 for a while so there were a ton 
of updates. Most of them finished fine. Then I started getting errors 
like 
http://security.ubuntu.com/ubuntu/pool/main/thunderbird/thunderbird-local-en-us_52.7.0+build1-Oubuntu0.16.04_all.deb. 
404 not found. Seems most of them have pool in the url. Probably have a 
dozen such errors.

Thanks,  Jim