The web application can connect to most email providers. In most instances the email servers are hosted by a domain provider (e.g GoDaddy, LiquidWeb, Outlook.com, etc).
We have discovered that some firms only allow SMTP server access on their email servers with the IMAP servers disabled. In these instances the easiest fix (rather than comprise your IT security) is to use a custom email on our domain [ @SimpleOptOutCompliance.com].