I have been struggling to find settings that work. So I hope this helps someone.
REQUIREMENTS: fully working install of helm email account using Thunderbird client already (& some familiarity with thunderbird for account setup)
This process will take time according to the emails you may have. In mine, it was (to me) quite considerable. Many hours were used but it was simple of set & forget till the process completes. Expect to just set this, go to bed & continue the following day DEPENDING on emails you might wish copied.
1.) Log in to your old Gmail account via google.
2.) (in GMAIL) go to Gear Icon> QUick Settings > See all settings
3.) Click Forwarding and POP/IMAP
4.) Under POP/IMAP, go to subsection “IMAP access:”
& be sure that STATUS says IMAP ENABLED: (if not toggle & save changes)
5.) Click Configuration Instructions on the bottom of the same page
6.) I used SSL suggestions for the SMTP instead of TLS (for some reason I could not get it to work in thunderbird with TLS)
7.) as stated, thunderbird settings for this page worked for me using SSL, but your mileage may vary)
8.) once you have successfully set up your GMAIL using thunderbird (through Configure manually), allow GMAIL to populate through thunderbird all of your old emails.
9.) Go to the folder of old GMAIL, go to the windowpane of emails on right (if thunderbird still left in default view modes) and highlight emails you desire copied (or click CTRL+A for all)
10.) allow for thunderbird to process & then right-click on highlighted emails & go to “COPY TO:” Selection in popped up menu
11.) Target the Same folder in your helm account in this menu and the client will process the copy.
As stated above, this might take some time. But I hope this is a little clearer to those who might need it.