Under Services > Email > Settings you can configure some additional settings for the email system.

The postmaster is the user who receives messages regarding errors and undeliverable messages. There is a system-wide standard postmaster and it is possible to set an individual postmaster for each domain (under Services > Email > Domains). Emails sent by the system use the postmaster's address as the sender. If you have not defined a domain in the system, you should enter a valid address at "External address of the postmaster", as many servers do not accept emails from invalid senders.

Emails cannot be of unlimited size as they must be temporarily stored and unpacked for further processing (e.g. virus scan, etc.). The spool partition whose space is limited is used for this. 100 MB has proven to be a reasonable limit. Very few systems will accept or send larger emails.