There are several ways that email can be added/accessed on your domain name. Email forwarding, POP3 Mailbox Accounts, IMAP Mailbox Accounts, Google Suite/Apps, Microsoft Office 365, Server Name / MX Record change, Web Mail
We have a limit of 20MB on attachment sizes (or total email size) for all emails that are sent or forwarded via our SMTP servers. There is no limit on incoming email. That said, every SMTP/POP server in the chain of email delivery may impliment different rules, and there is no guarantee of an email with > 5MB attachment succeeding.