Your file size matters when sending emails, especially through ESPs like Postmark, as large messages can lead to delivery issues and even spam filtering.` `Postmark has a limit of up to 10MB for message sizes, including headers and attachments, but larger messages can be flagged for spam by mailbox providers like Apple Mail, Outlook, Google Apps domains, and Yahoo.` `Message size is directly related to email deliverability, with most messages under 100KB seeing the same acceptance rates across all mailbox providers, while larger messages may trigger spam filtering or delivery issues.