I've been trying to get vcenter to send out emails when alerts are triggered but none get sent. I get the following error from the server logs:

2009-06-03 09:10:56.337 'App' 97643408 verbose VpxdMoEventManager Event6485: Alarm IIS Alarm on hqfcriis01 failed to send e-mail to 55677@mydns.uk;
summary = "'From:' email address for email alerts (for example, alarm@company.com)",
key = "mail.sender",
key = "mail.smtp.port",
key = "mail.smtp.server",
key = "mail.sender",
key = "mail.smtp.port",
key = "mail.smtp.server",
action = (vim.action.SendEmailAction) {
action = (vim.action.SendEmailAction) {
action = (vim.action.SendEmailAction) {
action = (vim.action.SendEmailAction) {
label = "Send E-mail",
summary = "Action to send an e-mail",
key = "vim.action.SendEmailAction",
key = "vim.event.ScheduledTaskEmailCompletedEvent",
formatOnDatacenter = "Task {scheduledTask.name} on {entity.name} sent e-mail to {to}",
formatOnComputeResource = "Task {scheduledTask.name} on {entity.name} sent e-mail to {to}",
formatOnHost = "Task {scheduledTask.name} on {entity.name} sent e-mail to {to}",
formatOnVm = "Task {scheduledTask.name} sent e-mail to {to}",
fullFormat = "Task {scheduledTask.name} on {entity.name} in {datacenter.name} sent e-mail to {to}",
key = "vim.event.ScheduledTaskEmailFailedEvent",
formatOnDatacenter = "Task {scheduledTask.name} on {entity.name} failed to send e-mail to {to}: {reason.msg}",
formatOnComputeResource = "Task {scheduledTask.name} on {entity.name} failed to send e-mail to {to}: {reason.msg}",
formatOnHost = "Task {scheduledTask.name} on {entity.name} failed to send e-mail to {to}: {reason.msg}",
formatOnVm = "Task {scheduledTask.name} failed to send e-mail to {to}: {reason.msg}",
fullFormat = "Task {scheduledTask.name} on {entity.name} in {datacenter.name} failed to sent e-mail to {to}: {reason.msg}",
action = (vim.action.SendEmailAction) {
2009-06-03 09:41:38.220 'App' 89222032 verbose ScheduledTaskActionLRO Failed to send email. Error:Application Error - Invoke() command must exist/be executable: "/usr/sbin/ssmtp"
2009-06-03 09:41:38.235 'App' 89222032 verbose VpxdMoEventManager Event6586: Alarm state on hqfcrintranet04 failed to send e-mail to 55677@mydns.uk;
action = (vim.action.SendEmailAction) {

When looking on the CentOS serve for /usr/sbin/ssmtp it doesn't even appear to be installed - which is why the emails aren't being sent out, anyone else finding this?

Thanks