"Successfully sent" is misleading. It simply means your message left ICON, not that it reached the recipient(s). You get the "successfully sent" message even when you enter incorrect addresses or use commas instead of semi-colons (;) to separate the recipients' names (ICON doesn't recognize the comma as a separator). It might help to copy yourself on the message, in all groups of 50-100, to know that the email you sent actually cleared the email server.