You can't sync mail between devices with a POP setup. All you can do is keep messages in the Inbox after download so they can be downloaded to other devices. But no other folders are accessible, and when a message is deleted or moved on one computer, the actions are not replicated on other devices.

IMAP is the standard way to sync mail between devices: desktops, tablets, phones etc.

Thanks for that information!

Thunderbird = TB.

• in POP based access your emails are downloaded into your local computer, and emails are deleted from remote mail-server.

• in IMAP based access your emails are kept into remote mail-server & a copy is downloaded into local computer.

• if you do not want to keep emails in remote-mail server, for whatever/various reasons, then: • you may use a sync software (in any one of the computer) , to keep two different folders in two different computers, in sync-with / same-as each others. • sync software list: https://en.wikipedia.org/wiki/Comparison_of_file_synchronization_software

• in sync software , you will need to use specific network protocol , allow those protocol inside your home/office's nework-router. • do not use such sync software that uses remote servers to sync your data, as that allows them to SPY-ON (STEAL data) from you. • in sync software in one computer, choose specific folder as source , and choose the other computer's similar destination folder , to sync.

• in TB > goto Tools > "Account Settings" > select any mail-account > Server-Settings > scroll down to "Local Directory" / "Local Folder" line, here you will see exact folder/directory location inside that computer, where emails are located, and where your TB-profile is.

• in TB , all emails under a specific folder in IMAP/POP are normally kept combined into a single file inside the TB, by default. • so you cannot move individual email normally, unless you apply below solution:

• inside the TB main menu > Options/Preferences/settings > Advanced > choose "File Per Message" , instead of "File Per Folder". above option may be inside "General" in some versions.

• above option will LATER allow TB to save each email as a single/individual file , & this is friendly for sync.

• restart TB.

• after changing that option, all earlier emails will not suddenly become single/individual file. • mail-account have to be re-created after setting that option.

• how to do it safely ?

• inside the "Local Folders" , create sub-folders for your mail-account folders like these: mail-account-01-Inbox mail-account-01-Sent ... mail-account-02-Inbox mail-account-02-Sent ...

• make backup, that is, make COPY of all emails into correct corresponding sub-folders under the "Local Folders". ( do not back/copy, before setting that option )

• then in TB goto "Account-Settings": • goto "mail-account-01": rename email-address into a different-one which does not exists , change mail server-name into a non-existent server name, etc. ( Do not any delete mail-account, yet , untill all transfer+transformation are done ) • create new mail-account in TB, with correct credentials. • check if earlier emails are back or not , if not then copy from backup that you made earlier into sub-folders under the "Local Folders". • restart TB. • check for emails again. • now all emails under this mail-account-01 are transformed into single individual file. • its also helpful to scan+quarantine infected emails this way. • try to keep less than 1000 files under one single folder, if necessary create many sub-folders with sequence number in end, like: 01 , 02 , 03 , 04, etc . that will help to perform various operations bit faster.

• in above way, transform your ALL OTHER EARLIER mail-account's all emails into single/individual files , one by one. OR, do above steps for only those mail-accounts , which needs to be synced across different computers.

• restart TB.

• at this point, of steps, you can obtain Sync software, prepare your computer's LAN settings, etc for the sync software . specify two different folders in sync software, from two different computers.

ANOTHER WAY TO SYNC: • in one/1st computer , follow the steps in above which are for Transform combined emails file, into single individual file per email. • in 1st computer you have to enable network-sharing . create a subfolder there for TB mail-accounts: \\COMPUTER-NAME-01\SHARE-NAME-01\Shared-Folder-For-TB-MailAccount-01\ ... \\COMPUTER-NAME-01\SHARE-NAME-01\Shared-Folder-For-TB-MailAccount-02\ ...

• in one/1st computer, which you enabled network-sharing , in TB goto TB's specific mail-account's "Server Settings" & scroll down > then write down that mail-account's "Local Directory" folder location. • exit TB. • with Windows-Explorer, copy all items from that "Local Directory" folder location into below SHARED location: \\COMPUTER-NAME-01\SHARE-NAME-01\Shared-Folder-For-TB-MailAccount-01\ ... • start TB in 1st computer, allow it to adjust with new folder location . check if your emails/folders appeared or not. • exit TB.

• in 2nd computer , follow the steps in above which are for Transform combined emails file, into single individual file per email. • in 2nd computer you have to enable network-sharing . create a subfolder there for TB mail-accounts: \\COMPUTER-NAME-02\SHARE-NAME-02\Shared-Folder-For-TB-MailAccount-01\ ... \\COMPUTER-NAME-02\SHARE-NAME-02\Shared-Folder-For-TB-MailAccount-02\ ...

• in 2nd computer, which you enabled network-sharing , in TB goto TB's specific mail-account's "Server Settings" & scroll down > then write down that mail-account's "Local Directory" folder location. • exit TB. • with Windows-Explorer, copy all items from that "Local Directory" folder location into below SHARED location: \\COMPUTER-NAME-02\SHARE-NAME-02\Shared-Folder-For-TB-MailAccount-01\ • start TB in 2nd computer, allow it to adjust with new location . check if your emails/folders appeared or not. • exit TB.

• at this point, of steps, you can obtain+install Sync software , prepare your computer's LAN settings, etc for the sync software . specify two different SHARED NETWORK FOLDERS (which are used by TB) in sync software, from two different computers.

GENERAL INFO & WARNING/CAUTION: syncing individual email files (not combined email files) , generally works better with POP access based mail-accounts in TB. some sync software will sync individual email files instantly, (depending on your setting), but those may not show up in TB, instantly, unless a refresh is done inside TB, or, better is, you restart TB, after each few hours. shown process/steps in above are not suitable for mail-accounts, which are configured/set to be accessed via IMAP. shown process/steps in above are not suitable for mail-accounts, which are using combined email files (aka "File per folder" option).

There is a difference in technically possible and practical. It is a good idea to not recommend to anyone that they try and synchronize any files and folders between profiles. As Thunderbird has no internal controls over file contention accidental use at the same file as it is synced leaves corruption or a second file in it's wake. So one profile suddenly starts using inbox-2... then the settings sync and either loose inbox-2 or force migrate to it loosing all the old mail that was in inbox. Either way mail gets lost. Another issue that crops up here fairly regularly as questions about setting no saving (especially in session.json) caused again by sync issues as well as things like having 50 popstate.dat, (popstatedat-X.dat beng the format), files and in one instance having them appear as folders within Thunderbird.

Until such times as Thunderbird is changed to understand about contention, syncing will be an exceedingly poor idea. Use of the portable version is my suggested workaround for those that just have to use the same profile for some reason. https://portableapps.com/apps/internet/thunderbird_portable Otherwise syncing is for the very technical with very good backups.