The total message size is limited to 20,480,000 bytes, or approximately 19.5MB. This includes all the headers, body, and attachments. That means that the maximum attachment size depends on the size of the rest of the stuff in your message.
Also keep in mind that the size of your file on disk or in memory may not be the same as the size of the file once it is attached, as files must be encoded as text to be sent. You can expect an increase in size of up to 30%.
Attachments are in and of themselves potential risks when sending email. Email filters are more likely to prevent delivery of a message that has an attachment due to the risk of viruses.
A good alternative to attaching a file to your message is to place it on a page of your website, and then provide a link to that web page within your message body.
SendGrid has partnered with Digioh, a digital download distribution service. This allows you to send files up to 2GB in your SendGrid Marketing Emails. Digioh also allows you to track individual downloads and prevent non-subscribers from downloading files.
Digioh is integrated with SendGrid’s Marketing Email solution, so you can be up and running in no time.