01 Start with a message you can identify.
Fictional example: Mira knows a message titled “Saturday rehearsal” is in her desktop Inbox. She finds the same subject, sender and date in her provider’s webmail. That gives her a concrete server-side reference for later comparison. A total message count alone would be a weaker check: views can filter, thread or hide messages differently.
IMAP clients access mail held on a server. Changes can synchronize between clients, while offline copies depend on download settings. Check your desktop account’s Server Settings to identify IMAP or POP. Thunderbird’s IMAP explanation.
02 Treat a local archive as its own object.
Fictional example: Omar’s 2018 correspondence appears under Local Folders on his laptop. Searching webmail does not find his chosen message. He records it as “local copy found; server copy not found,” rather than assuming another phone app will recover it.
A Thunderbird desktop profile holds messages and other account data. Keeping that profile separate from the program is not itself a backup. Before a storage change, keep a separate, verified copy using the official profile and backup guidance. A profile copy is not a promised iOS import format.
03 For POP, investigate before moving.
Fictional example: Jules has a POP account with years of downloaded mail. Recent messages still appear in webmail; older ones do not. The account label alone does not reveal which copies remain on the server. Jules records an old and a new sample separately.
Moving from POP to IMAP requires provider support and a deliberate transfer of any local-only messages you want on the server. Consult the official migration procedure, check storage limits, and retain your backup. This preparation exercise does not ask you to delete, disable or convert an account.
Your useful next observation
Write down one folder, one non-sensitive subject, its date and the place you actually found it. Do not record passwords. For a clean display comparison, use the original synthetic mail samples instead of private correspondence.
If you also need a separate browser session to inspect your provider’s webmail, WebTraceCheck offers temporary cloud browsers. That checks a web view, not a native iPhone app. Its cloud browser requires sign-in; session files are discarded. For sensitive mail, your own trusted device remains the appropriate place.