You have some PST files from Outlook that hold emails and some of those emails have attachments that are larger than 25 MB. Generally, this can become an issue when you go to move them into Thunderbird because many of the manual ways you can do that will require you to use IMAP, and IMAP has a limit of 25 MB attachments.
Two Ways to Handle This
1. Manual Way (Free but Limited)
Set up an IMAP email account (e.g. Gmail)
Add that account to Outlook.
Move emails from PST into the IMAP account in Outlook.
Then add the same IMAP account in Thunderbird. The emails will be synced to Thunderbird.
But here is the issue: in the event an email has an attachment larger than 25 MB, Gmail (and other IMAP servers) will not accept it, therefore, those emails will not transfer correctly.
Basically, the manual way will only work well without any big attachments.
2. Technical Way (Mailvita PST to MBOX Converter for Mac)
Instead of utilizing IMAP, you can use a software application that will convert PST to an MBOX format (using Thunderbird's MBOX format).
Mailvita PST to MBOX Converter for Mac does this job.
The application doesn't hinder attachment size - whether the email has a 50 MB email or a 100 MB file, it converts it without issue.
The application maintains the folder structure, formatting for the emails, and all the related metadata as required.
Once the conversion is completed, you can simply import your MBOX file into Thunderbird (using either the ImportExportTools NG add-on or Thunderbird's import option).
Straightforward Comparison
If you utilize the manual IMAP method of importing Mail into Thunderbird, anything larger than a requested 25 MB attachment will not transfer to Thunderbird. To avoid this problem you can utilize the Mailvita PST to MBOX Converter for Mac, which will directly convert the PST to an MBOX file and returns no data loss including even large attachments to be safe and secure for Thunderbird.
Two Ways to Handle This
1. Manual Way (Free but Limited)
Set up an IMAP email account (e.g. Gmail)
Add that account to Outlook.
Move emails from PST into the IMAP account in Outlook.
Then add the same IMAP account in Thunderbird. The emails will be synced to Thunderbird.
But here is the issue: in the event an email has an attachment larger than 25 MB, Gmail (and other IMAP servers) will not accept it, therefore, those emails will not transfer correctly.
Basically, the manual way will only work well without any big attachments.
2. Technical Way (Mailvita PST to MBOX Converter for Mac)
Instead of utilizing IMAP, you can use a software application that will convert PST to an MBOX format (using Thunderbird's MBOX format).
Mailvita PST to MBOX Converter for Mac does this job.
The application doesn't hinder attachment size - whether the email has a 50 MB email or a 100 MB file, it converts it without issue.
The application maintains the folder structure, formatting for the emails, and all the related metadata as required.
Once the conversion is completed, you can simply import your MBOX file into Thunderbird (using either the ImportExportTools NG add-on or Thunderbird's import option).
Straightforward Comparison
| Method | Can it move attachments over 25 MB? | Easy to Use? | Needs Outlook? |
| Manual (IMAP) |  No, attachments >25 MB are blocked | Complicated |  Yes |
| Mailvita Converter | Yes, no size limit | Very easy | No |
If you utilize the manual IMAP method of importing Mail into Thunderbird, anything larger than a requested 25 MB attachment will not transfer to Thunderbird. To avoid this problem you can utilize the Mailvita PST to MBOX Converter for Mac, which will directly convert the PST to an MBOX file and returns no data loss including even large attachments to be safe and secure for Thunderbird.