The reason for this is that I am tired of dealing with sendmail, spammers, and everything associated with it. When I am not dealing with security I am dealing with sendmail jumping to 60% CPU usage and other random scenarios. I give up being a mail server admin.
Now, sending mail through SMTP with Google Apps is allowed, although it is restricted to 500 mails for the free accounts and 2000 for business. For a busy forum, 2000 is too little, and I easily exceed that limit.
Google refuses to upgrade the limit, and the suggestion from them is sending the email from different accounts. It is valid to create a email@example.com account, mailer2@, mailer3@, etc and when I reach the limit from one account I could start sending the email from the other.
While I do not expect XenForo to manage my email, it would be really useful if I could setup multiple SMTP accounts, and the mail system would just choose one at random, this would give me a reasonable distribution to avoid reaching the quota.