Any reason why user(username,"email") will work sometimes, and i

Are you sure that a username is always populated in your variable when you call the user() function? Also, are you sure that every username is associated with an email address?

user name is populated, and get exception nested exception is:

          com.sun.mail.smtp.SMTPAddressFailedException: 550 5.1.1 <username>... User unknown

We sometimes have the problem that some new users do not get properly propagated to all exec engines. The checkengine script can check for this and also fix it. Maybe you give it a try.

If the error you are seeing when you believe the user(username,"email") function is not working is:

com.sun.mail.smtp.SMTPAddressFailedException: 550 5.1.1 <username>... User unknown

then this is not an issue with the expression or with Appian. An exception of type"SMTPAddressFailedException User unknown" is generated by the mail server itself.

Please contact your Mail Server administrator and verify if the server isn't permitting sending on to other domains, most likely the addresses that are causing the error are outside the permitted domain configured in the mail server; nothing to troubleshoot at the Appian level but at the mail server level with you administrator.