Given IMAP is not a "file system" it can cope with messages being added in smallish numbers, but will almost always mess up on large quantities of mail. This is not a Thunderbird problem, but really a limitation on the whole IMAP protocol that affects all mail clients. The current trend to trying to get mail stored on someone elses computer by using a mail client is basically upload thousands , or tens of thousands of email is doomed to long term failure.

The bottom line is log into your google account using a web browser, configure the account to get your mail from your ISP via it's pop service and you will not need to fiddle around trying to make your ISP mail appear in Google at all via Thunderbird.

As for your old mail, export it as EML files and upload it to Google drive and search it as a collection of files.

Thanks for the answer although it wasn't what I was hoping for. Old emails are really the issue. I find that I am searching my archives on a regular basis either to find something that I wrote to modify and resend or something that I sent to someone else who now can't find it. I may just install Tbird and all of its archives on my new PC just to be able to search for stuff.

While I cannot suggest anything more about putting all your old e-mails on Gmail, I strongly suggest that you do not solely count on Gmail's services.

If it is important to you, back it up. ie. Also have a copy on your computer. That is why Thunderbird has "Local Folders". So you can copy (or move), e-mails from the IMAP to be saved on your computer in "Local Folders".

If your intention is to avoid using Thunderbird entirely in the future, you might want to look at MailStore Home. It is a free program for home users. You can use it to archive your old e-mails from Thunderbird (or other e-mail programs), and still be able to search and find them in MailStore. Later, if you want to, you can export the e-mail from MailStore to another e-mail program. MailStore can also be used to archive e-mail that is only in webmail, like Gmail.

Thanks for the info on MailStore. I will look into it. I can't say right now if I will be leaving Tbird permanently but I definitely know that I need email access on the phone and don't want the complexity of multiple email accounts.

We ignored the original reason why you were going to use Gmail, and that was to be able to access your e-mail on your phone.

Your ISP probably also offers the ability to access your e-mail account using IMAP, and that can be set up on your phone. It will not have access to all your old e-mail on your computer, but it will work.

You can also then switch to using IMAP for your ISP e-mail address in Thunderbird: Switch from POP to IMAP account

IF your mail is on a mail server, you can tell the secondary mail account to collect it. This was one of the early selling points for Gmail and all the big free players have adopted it.

So you create a primary account and have it collect all the mail for your others via POP.

The drawbacks are; You will have difficulty storing replies on a single server, gmail will let you reply using other email addresses if you register them, but it has to be set up. POP only knows about the inbox. So mail send from the account will not be picked up.

Matt,

Thanks for the suggestion but all of my mail is downloaded to my "Local Folders" from my ISP's POP3 server.

Bruce,

My ISP is Time Warner. They have a hard enough time handling just POP email. I am pretty sure that they don't support IMAP or if they did the storage would be so small as to be nearly useless.

nah TWC does IMAP https://www.timewarnercable.com/en/support/faqs/faqs-internet/e-mailacco/incoming-outgoing-server-addresses.html

You will probably find it better than their pop service as most mobile/cell users use IMAP.