That is a "feature" of Gmail. I suggest you either use another mail provider or address your questions about the feature to Google.

Is there a reason you're using a POP account? An IMAP setup on all your devices would probably work better.

http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

POP works better for my needs as I have different uses for each device so with POP I am able to be selective as to which device retains a particular email.

I was not aware that this is a "feature" of Gmail as only Thunderbird seems to cause this issue. If I don't use Thunderbird then this issue doesn't arise....

HomerBird said

I was not aware that this is a "feature" of Gmail as only Thunderbird seems to cause this issue. If I don't use Thunderbird then this issue doesn't arise....

For POP, the server is not supposed to have any idea if a particular device has retrieved mail.

The process works by opening the connection and asking what mail is there. This is compared against what Thunderbird has downloaded and left on the server (popstate.dat file). Thunderbird then requests mail after number XXXXXX

So if the server is keeping records, marking the mail as downloaded or read then it is a "feature" of that particular server. The RFC is here if you want to check for yourself. https://tools.ietf.org/html/rfc1939

Thanks for replying Matt. Perhaps I am not explaining the situation correctly. If I have email in Gmail, I can get the email on my iPhone and it remains in Gmail. I can then also get that same email in Thunderbird. However, if I reverse the process and get the email using Thunderbird first, I am now not able to get the email on my iPhone any longer, even though it I am still able to see it in Gmail. I do have Thunderbird configured to leave mail on the server (Gmail), which it does do, but if I retrieve the email using Thunderbird initially, I cannot retrieve it on my iPhone even though it is still in Gmail.

Sorry if I repeat myself but I just want the situation to be clearly understood.

is the mail being marked as red on the gmail server? I have heard of iphones not getting mail that is marked as read.

I suggest you go here https://mail.google.com/mail/u/0/#settings/fwdandpop

Make sure that mail accessed with POP is left in the inbox and not marked as read.

No, not being marked as read, and the mail is left in the inbox. That's why I am perplexed.

Perhaps you need to ask apple why the mail is not retrieved.

It also cannot be retrieved by other email clients as well after I retrieve it with Thunderbird - that is the quandry. I would doubt Apple has anything to do with it.

I have a copy of a simple email client, Popcorn v1.99.6, which I have used for years to very quickly and simply remove mail from the server. Popcorn also cannot see email on Gmail after retrieval with Thunderbird. In fact, any email headers seen with Popcorn will subsequently disappear from view after I check for email with Thunderbird. I am somewhat tech savvy and this is one of the weirdest things I have dealt with.

The common denominator sees to be Thunderbird, as if I take T'bird out of the equation, all is well.

I really do appreciate your input.

I have just found out about Gmail's "recent" mode. I am thinking that this may be at the root of the problem.

"Recent Mode" is referenced in the following: https://www.lifewire.com/get-gmail-in-all-programs-and-devices-with-recent-mode-1172096 and http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

I am going to try to tweak things, when I get a chance to, and see if this will help. I am hopeful.