Your storage in quota is with your mail provider (the company those name comes after the @ in your email address) and is simply reported in Thunderbird as a courtesy.

But historically there have been issues with mail actually getting deleted from the server when moved in Thunderbird which might affect you. I suggest you use the webmail from your browser to check exactly what is in the account on the web.

Additionally the quota is not necessarily mail only. For example Google send out total usage of allocated space rather than just mail as they do not differentiate the source, so it may be your photos that are backed up to Google, or your phone backup that is filling your quota, not mail. That is if it is a google account. Each provider is just a little different.

Thank you for the information. It is Google that I use for email and photos, but when I check my storage I have like a terabyte or more left. I also use Dropbox for storage of things, wondering if I could copy my photos, which I do have a lot, into Dropbox and lower my Google usage quota, or will they still remain in Google? I also have considered an external jump drive for copying photos out of Google, but can't find a way to move them, just the option to delete them.. I believe it is the combination of photos and email that is generating this message. I appreciate your advice and assistance.

All the quota message says is you are at 75% it does not say you have to do anything now or even in the next 12 months.

The Google page offers a color coded graph under the usage to show what is using the space. What you choose to do is up to you.

But one of the things everyone has to start thinking about is actually deleting some mail from their online mail accounts, or perhaps storing it locally in a removable drive if they can not bear to get rid of it. Google takeout allows the downloading of your entire google email store by label which can be archived on a hard disk for later viewing then the data can be deleted from the cloud storage. The downside is you are responsible for the backup and storage of your own data.

Thank you so much for all the information! Just weird that that quota message just appeared after years....I will look into all the things you suggest. Appreciate you taking the time to respond to my question. Jan