The maximum email size for the BT Mail service is set at 25MB. However, you may find it hard to send attachments that are bigger than about 20MB because they'll need to be encoded.
The maximum email size for BT Mail or BT Premium Mail is set at 25MB. However, you may find it hard to send attachments that are bigger than about 20MB because they'll need to be encoded, and this adds about a third to their original size. (For example, a 1.5MB attachment is around 2MB once it's been prepared for sending).
If you need to send large files, you can make them smaller by "zipping" or compressing them first. Single or multiple files can be made smaller with compression programs such as Winzip or Stuffit. You can often reduce file sizes quite considerably.
Transferring files online
As an alternative to emailing large files, you could use an online backup and storage service like BT Cloud. This provides an easy way to share extremely large files - up to 1Gb - with your family and friends.
Even better, BT Broadband customers get BT Cloud free.
Other services and tools
There are a various services and tools that allow you to transfer extremely large files. Try searching the web for "file transfer tools" and you should be able to find one for free.