In these days of high-speed Internet access via cable systems, we think little about sending large attachments with our emails. We can send and receive them easily, so we forget that not everyone can.
The maximum email size you can mail depends on what YOUR ISP allows and what your recipient’s ISP allows.
Your recipient may have a limited mailbox size (often these are in the 5MB to 10MB range, but I’ve heard from a local DSL user that their DSL provider has a 1MB mailbox size limit for its cheapest accounts. Even one full-size photo could be too much for those mailboxes (they need a Gmail account with a 2+ GB mailbox).
Another factor that affects attachments is that, files that you send as attachments are not sent in the same format via email. They are sent in a much larger form. We don’t notice it happening, but the attachment is encoded using a standard system called UUEncoding.
Since our email systems (programs AND mail servers) are incremental upgrades from old email standards in use for over 20 years, not only is data sent as 7-bit data (instead of normal 8-bit data), it is translated into PRINTABLE characters for the transmission.
In effect, an attachment is expanded to 40-60% larger than the original file. You may see this number as 30% or 80% or even over 100%. They can all be true — the expansion depends on the compressibility of content you are sending.
The UUencoding system, like all other encoding systems (for example, zip files), looks at the repeats in the data. If an individual byte in a file is repeated 100 times, the encoded file doesn’t translate it 100 times — it stores a “repeat the following 100 times,” the byte (or set of bytes to be repeated, and “this is the end-of-repeat” marker.
Back to email attachment sizes — if the file you’re emailing has already been compressed (examples, a JPG photo or image file or a zip file), there is nothing left to optimize.
With nothing left to compress, the UUencoding can only make the file larger, as it converts each data byte of the file (each byte can have a value from 0 through 255, most of which are not printable) into a combination of one or more printable characters.
Since there are 256 values being crammed into only 95 printable characters in the standard ASCII character set, you can see that this process often results in larger attachment sizes than the size of the original file.
So, when you want to email photos or videos to your friends, think first. If they have high-speed Internet connections, they can download the larger message easily (if their email box gives them enough space). If they’re using dialup connections or if their ISP does not give them a big mailbox, they may not want the big files.