Have you checked the sort criteria selected? Maybe you have the mails sorted by arrival, rather than date, or oldest first.

The below setting can limit message fetching but would stop you seeing older messages, not new ones. Settings > account > fetching mail > fetch messages from

I'm presuming your familiar with the app and do have the sync setting enabled for each applicable folder.

If you send yourself a new email, is it not received in the mobile app?

Have you checked the sort criteria selected? Maybe you have the mails sorted by arrival, rather than date, or oldest first.

No. It's not the sorting. I've always had it configured for date/descending.

The below setting can limit message fetching but would stop you seeing older messages, not new ones. Settings > account > fetching mail > fetch messages from

No, it's not the fetching either. I had it on "any time". Tried "last 3 days" and now the inbox is empty on the phone, whereas on the PC I have >50 from the last 3 days...

I'm presuming your familiar with the app and do have the sync setting enabled for each applicable folder.

Curiously, I've used TB for a very long time on the PC. But on the phone I've mostly used the Gmail app alone. I've used K-9 in the past though. So, I'm not new to TB on Android but I'm not that familiar with advanced settings like sync-per-folder and stuff like that.

If you send yourself a new email, is it not received in the mobile app?

I've sent a message to my Infomaniak address and it got through and it's showing up on my Inbox. But if I send a message to my Google mail that is redirected to the Infomaniak email account, the message shows up on my PC but it doesn't reach my phone.

Update: I got it working! Settings>Account>Fetching Mail>Local folder size That was set for 100. Set it for "all messages" and now my TB is a bit sluggish but I have all the messages on all folders. What can I learn from this? Do I really need to have it set up for "all messages"? It doesn't make sense...

The number of messages you need the app to hold depends on your use case.

For me personally, the mobile platform is best suited to reading new emails and short replies. Not sure what the use case is for holding years worth, or hundreds of emails on a mobile. How likely are you to read or reply to them. Each to their own I guess.

In any event, the app has known performance degredation with more than 500 messages so I'd recommend setting the folder size to 500 max, lower if you can. See the developer comments about this linked below.

https://github.com/thunderbird/thunderbird-android/issues/10373#issuecomment-3810077139

Sync setting If you have created any imap folders / labels on an account, they don't automatically sync by default. You need to turn sync on for each desired folder.

To check a folder...

1. Open the account list
2. Tap an account
3. Tap manage folders (bottom of list)
4. Tap a folder name
5. Toggle "enable sync" to ON

For me personally, the mobile platform is best suited to reading new emails and short replies. Not sure what the use case is for holding years worth, or hundreds of emails on a mobile. How likely are you to read or reply to them. Each to their own I guess. </blockquote> Fully agree. Actually, the last 100 messages would be more than enough in my personal usecase, even because there's always the "load up to 100 more" button if I need it. I only tried the "all messages" option to see if the missing messages appeared. And they did.

Sync setting If you have created any imap folders / labels on an account, they don't automatically sync by default. You need to turn sync on for each desired folder.

I didn't know that. But it actually makes sense.

What doesn't make sense is that using the last 100 messages option is still not working. It is still not showing me the latest 100 messages in my Inbox folder. It works on the computer but it doesn't work on the phone.

How does TB account for which messages are the latest? Does it use a "time of arrival"? Or does it use the message timestamp that can be seen if we show the message headers? Because if it uses the time of arrival, maybe it makes sense. I did move the old messages from the mbox file to the Inbox just 24h ago...

The app was built to comply with the imap specification (RFC 3501).

The "Server Internal Date Message Attribute" is used to determine which messages are fetched. This will likely hold the "Arrival Time" when you imported the mbox file to the servers inbox.

RFC 3501 https://datatracker.ietf.org/doc/html/rfc3501#section-2.3.3

2.3.3. Internal Date Message Attribute

  The internal date and time of the message on the server.  This
  is not the date and time in the [RFC-2822] header, but rather a
  date and time which reflects when the message was received.  In
  the case of messages delivered via [SMTP], this SHOULD be the
  date and time of final delivery of the message as defined by
  [SMTP].  In the case of messages delivered by the IMAP4rev1 COPY
  command, this SHOULD be the internal date and time of the source
  message.  In the case of messages delivered by the IMAP4rev1
  APPEND command, this SHOULD be the date and time as specified in
  the APPEND command description.  All other cases are
  implementation defined.

Grateful for the clarification and for the RFC snippet. I have a couple of questions, though. The first question is, do you think this is really the root cause for my problem? The second is, why exactly would TB for PC work different from TB for Android? Maybe because TB for PC shows all the messages regardless of their amount and of their timestamp? The last, what could I do to try and mitigate the issue? What would you do if you had to combine old emails retrieved from an mbox archive into a new IMAP account in order to avoid/workaround the problem I am experiencing? Thanks in advance. Cheers

[The first question is, do you think this is really the root cause for my problem?]

It's one possible explanation but couldn't say for certain. I've never tried importing from an mbox file. A perplexity AI search advised the date applied is down to the behavior of the import tool.

When you import from an mbox file, most tools use IMAP APPEND to push messages into the new mailbox and either:

• Pass the original “received”/header date as the APPEND date argument, in which case Internal Date will match the original server’s receive time.

• Or omit a date argument, in which case the server sets Internal Date to “now”, i.e., the time of import

Did your import tool offer any settings or options ?

[The second is, why exactly would TB for PC work different from TB for Android]

That's comparing apples to oranges. TB desktop and TB Android (K-9) have different heritages and code bases. Your supposition is reasonable.

Q1) Are your missing emails large in size. Have you tried increasing the "fetch message up to 32KiB" setting to a larger size?

Q2) Can you try changing the setting "Sync messages from". Set to "The last 3 months" and see if any missing messages download.

Did your import tool offer any settings or options ?

Actually, my import tool was TB on the PC. I imported the emails by opening the local folders in the same folder where I have the mbox files (Inbox, Sent, Archive in three separate mbox files), and then I used TB to move those local messages to the Infomaniak folders with the same name.

Q1) Are your missing emails large in size. Have you tried increasing the "fetch message up to 32KiB" setting to a larger size?

No, I'm not missing only large emails. I am missing almost all emails. I have a couple from 2025, a bunch from 2024, a bunch from 2023, some from 2022, only a couple from 2021 and 2020. That all adds up to about 20 messages in my inbox. Using the fetch up to 32KiB doesn't help. Some result.

Q2) Can you try changing the setting "Sync messages from". Set to "The last 3 months" and see if any missing messages download.

Doing that didn't change much. It added a bunch of messages, but not from 2026.

I have tried something different and it helped. It's almost a fix. But I did more than one change and now I don't know which change helped... This time around I erased all imported messages from Gmail on the Infomaniak account using the PC. Then I reimported the Gmail messages using TB on the PC. But this time I didn't open the mbox files browsing to their location using the local folders. This time I used a TB extension/addon named 'Import Export NG'. After the mbox files were visible on my local folders, then I proceeded to move them using TB on the PC to a Gmail folder I created with an Inbox and Sent folder inside. The imported messages are now inside of Gmail>Inbox and Gmail>Sent. My true Inbox now shows the right messages, but only those received on the Infomaniak account. The true Sent folder now shows only the Infomaniak account sent messages. All working properly. And if I open Gmail>Inbox and Gmail>Sent I can see the appropriate Gmail messages. Only the last 100 because that's how I have TB Android set up. Unfortunately this way I have Infomaniak messages separate from Gmail messages. But fortunately, this way it really shows the proper last 100. Would be cool if I could have them all mixed up inside the Inbox and Sent folders, though. Thanks mate. Cheers

Glad to hear you've made progress and at least have workaround :)