When sending or receiving e-mail messages always take into account the following:
The mail servers will allow up to 25 MB. It is however not recommended to send messages that large. The message you are sending can for example not be larger than the free space available in your own mailbox. You have to take this into account in case the message can not be delivered and is sent back to you.
Attachments in an e-mail message always have to be processed within the e-mail message itself. The encoding increases the size of the messages by one third. If an attachment is about 18 MB, the mail will take up about 25 MB.
For the 'Exchange' environment, the same limits apply.
When sending an e-mail via webmail, the attachment can not be larger than 8 MB. This restriction only applies when sending e-mail (new message, forward, reply, ...), not when receiving an attachment.
It is recommended to keep e-mail attachements as small as possible (e.g. by compressing them to a zip file) or to make files available in an other way.