As reported on the forum:
So I have recently moved over to LS5 and have just had the need to use the token based email with email invites. This has been a little problematic with the setup that works perfectly well on LS3.

Our email relay of choice is Amazons AWS SES service (Simple Email Service). This service works slightly different to other services as you:

1. Create an account in the region you wish to use
2. Setup the domains and the email addresses that are allowed to send through the service and verify them.
3. Setup a SMTP sending account which provides you with a 16 digit Username (not an email address) and the associated password.

So in the SMTP sending setup in LS 3 or 5 - you enter:
Default site admin (over writable at survey level)
Outbound Mail server / port
Username
Password
Encryption
When you try and enter the AWS SES username as given in the format ABCDEFGHIJKLMNOP and send a test message in LS5 it returns with the error that the username is not a valid email address.

Interestingly LS3 and every other service I use SES with, this works perfectly with the same setup. Also, if I change the setup in LS5 to use my O365 account which does have an email address as the login it works perfectly. Interestingly in my O365 I have a shared mailbox with the ability to send - every other service i have that uses this logs in with the username in the format "my@email-address.com/noreply" (noreply is the shared mailbox) and it works. But LS5 doesn't like this as its not a email address.

But if i use O365 with the smtp in the format of "my@email-address.com" - It works perfectly.

It would seem that the SMTP username is being used as the FROM field within LS5
Although very often the SMTP Username is an email address - it isnt always ie AWS SES or O365 Sharedmailbox - SMTP OAuth.