Common Email Conversion Errors and How to Fix Them

Why Email Conversion Sometimes Fails

Email conversion tools parse complex binary and text formats that have evolved over decades. While MailtoPst handles the vast majority of files without issues, certain edge cases can cause errors. Understanding the most common problems and their solutions saves time and frustration.

Error 1: File Is Corrupted

Symptoms

The conversion fails immediately or produces incomplete output with missing messages. The error report mentions “invalid header,” “unexpected end of file,” or “corrupted block.”

Causes

• The PST or OST file was not fully copied (interrupted download or file transfer).
• The file was damaged by a hard drive failure or bad sectors.
• The PST grew beyond its size limit (2 GB for ANSI format) and became internally inconsistent.
• An application crash occurred while Outlook was writing to the file.

Solutions

1. Verify the file is complete — check that the file size matches the original. Re-download or re-copy if necessary.
2. Run ScanPST.exe (Windows) — Microsoft’s Inbox Repair Tool can fix minor PST corruption. Run it multiple times if the first pass only partially repairs the file.
3. Try MailtoPst anyway — MailtoPst has built-in tolerance for minor corruption and may recover most messages even from damaged files. See How to convert a corrupted PST.
4. Split the file — if corruption is isolated to one section, extract the recoverable portions.

Error 2: File Too Large

Symptoms

Upload fails or the tool reports that the file exceeds the plan’s size limit.

Causes

• The file exceeds the free tier limit of 100 MB.
• The file exceeds the paid plan limit (50 GB for Enterprise).
• Network timeout during upload of a very large file.

Solutions

1. Upgrade your plan — the Standard and Enterprise plans support much larger files. See How to convert large PST files.
2. Split the PST — use Outlook to split a large PST into smaller files by date range or folder before converting.
3. Use a stable connection — for files over 5 GB, use a wired connection instead of Wi-Fi to prevent upload interruptions.
4. Compress before upload — while PST files do not compress much (they are already binary-packed), ZIP compression can help for collections of EML files.

Error 3: Unsupported or Unrecognized Format

Symptoms

The tool does not recognize the uploaded file or displays “unsupported format” message.

Causes

• The file extension was changed manually (e.g., renaming .ost to .pst).
• The file is actually a different format than expected (e.g., an MBOX file with a .pst extension).
• The file uses a proprietary format not based on standard PST/MBOX/EML specifications.

Solutions

1. Check the file extension — make sure it matches the actual format. Do not rename files to force a conversion.
2. Verify the format — open the file in a text editor and check the first few bytes. MBOX files start with “From ”; PST files start with ”!BDN” (hex: 21 42 44 4E).
3. Use the correct conversion — if you have an OST file, use OST to PST, not the PST converter.

Error 4: Password-Protected File

Symptoms

The conversion fails with a message about encryption or password protection.

Causes

• The PST file has a password set in Outlook.
• The OST file has profile-level encryption.

Solutions

1. Enter the password — MailtoPst prompts for the password during upload. Enter it correctly.
2. Remove the password first — if you know the password, open the file in Outlook and remove the password protection before converting.
3. See the detailed guide — How to convert a password-protected PST covers all scenarios.

Error 5: Incomplete Output (Missing Messages)

Symptoms

The conversion completes but the output has fewer messages than expected.

Causes

• Some messages in the source file are corrupted while others are fine.
• The source PST has orphaned items (messages not linked to any folder in the internal B-tree).
• Very old PST format (ANSI) with encoding issues.

Solutions

1. Check the conversion report — MailtoPst lists any messages that could not be extracted.
2. Run ScanPST.exe on the source file and convert again.
3. Try a different output format — if PST to EML misses messages, try PST to MBOX as the parsing path differs slightly.

Error 6: Upload Timeout

Symptoms

The upload progress bar stalls or the browser reports a connection timeout.

Causes

• Slow or unstable internet connection.
• Very large file size combined with limited upload bandwidth.
• Browser or network proxy interrupting long uploads.

Solutions

1. Use a faster connection — wired connections are more reliable than Wi-Fi for large uploads.
2. Disable VPN or proxy temporarily if they are causing interference.
3. Try a different browser — Chrome generally handles large file uploads better than older browsers.
4. Break the file into parts — for very large archives, split the source file and convert in multiple batches.

Frequently Asked Questions

What should I do if my conversion keeps failing?

Start by checking the file integrity (is it complete and not corrupted?), then verify you are using the correct conversion for the file type. If the problem persists, the conversion report provides specific error details that can guide troubleshooting.

Can MailtoPst recover messages from a partially corrupted file?

Yes. MailtoPst attempts to extract every recoverable message, even from files with structural damage. Messages that cannot be parsed are skipped and reported. See How to convert a corrupted PST.

Is there a way to test the conversion before paying?

Yes. The free tier provides 100 MB of conversion credit — enough to test with a small file or a sample of your archive. Try free — no credit card required.

Where can I get help with a specific error?

The MailtoPst support team can assist with specific conversion issues. Include the error message and file details when reaching out.

Upload your file now and resolve your conversion issues.