When trying to merge Contacts in Salesforce, you may run into this error if one of the Contacts you're trying to merge was a Soapbox Mailer Sender that no longer exists or no longer has an email address associated with it. In either case, you will be presented with the following error message on merging:
"first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Please choose a sender email address.: [sbm__sbx_EmailSender__c]"
While there are a number of remedies for this, including updating all the past Soapbox Mailer Campaigns to have a Sender with an active email address listed, the easiest approach may be to simply turn off the validation rule that requires a Sender to have an email address for a Campaign, do the merge, and then turn the validation rule back on. To do this....
1) Login to Salesforce
2) Go to Setup > Build > Customize > Validation Rules
3) Click the Edit link next to "sbx_EmailRequireSenderAddress"
4) On the edit view for the sbx_EmailRequireSenderAddress validation rule, uncheck the Active option, and click save
5) Do the Contact merge as needed.
6) Return to the sbx_EmailRequireSenderAddress validation rule and edit it so that the rule is active again