How to test message size and attachment limits?

Testing size limits prevents unexpected rejections:

EHLO SIZE parameter:

Server may advertise max size in EHLO response. SIZE=26214400 means 25MB limit. Not all servers advertise.

Progressive testing:

Send test messages of increasing size. Note at what size rejection occurs. Test with different attachment types. Document per-provider limits.

Common limits:

Many providers: 25MB. Some corporate servers: 10MB or less. Base64 encoding adds ~33% to file size.

Error indicators:

552 (message size exceeded). 5.3.4 (message too big). "Size limit exceeded" in error text.

Best practice:

Keep messages under 10MB for widest compatibility. Use file sharing services for large files. Test critical recipients' limits.

Size limits are the cargo weight capacity. Know them before you load up.