Symptoms

You may see the following error in Error Profiles for your worklfows "There is no connection string for the destination email address someone@somedomain.com"
 

Diagnoses

Despite this error message says "destination email address" more ofthen than not this error is caused by issues with your SMTP/EWS configuration and/or email address of your K2 service account (i.e. source email address). There is an existing feature request to improve error message information so that it will be more precise.
For example you may see such error after some setting on SMTP server side were changed and those are not updated from K2 side. For example in SMTP port was changed and you have to do the following for K2 servers farm:

1) Run K2 blackpearl Setup Manager in Farm Reconfiguration mode and enter all the relevant settings (this will update settings in K2 DB).

2) Run K2 Connection String Editor (located in: "C:Program Files (x86)K2 blackpearlHost ServerBinConnectionStringEditor.exe") on each of your servers it will update email connection strings in your K2 host server configuration file. It is also necessary when you need to setup authenticated SMTP connection you have to do it via Connection String Editor as K2 setup manager does not allow you to set those settings (there is an existing feature request for that).

3) Update Mail server field for the environment in your Environment Library as well as in String Table.

The same error message may also indicate that the issue is indeed with destination email address/user. In this case you have to confirm that the user is enabled in Identity tables and has correct email addrees. Sometimes it could be just a matter of identity refresh for affected user (i.e. URM service cache not yet picked up recent changes in AD for affected account).

Resolution 

See Diagnosis section above.