Capture the rejection and determine which system enforced the limit.
Check:
- original attachment size;
- final MIME message size;
- sender submission limit;
- gateways;
- recipient service limit;
- transport rules/policies.
Binary attachments encoded with Base64 become larger in transit, so comparing only the source file size with a nominal message limit can be misleading.
For very large content, use an approved file-sharing mechanism rather than repeatedly increasing email limits.