Because this issue still comes up from time to time, and it isn't always a situation where you want to terminate the process, I did some investigating into the database and was able to figure out a work-around...

This query will show email tasks that are stuck in this state.

SELECT *
FROM [LFForms].[dbo].[cf_bp_worker_instances] AS wi
JOIN [LFForms].[dbo].[cf_bp_worker_instance_history] AS history ON wi.[instance_id] = history.[instance_id]
JOIN [LFForms].[dbo].[cf_bp_steps] AS step ON wi.[current_step_id] = step.[step_id] AND wi.[current_process_id] = step.[process_id]
WHERE wi.[status] = 3 AND history.[finish_date] IS NULL AND step.[step_sub_type] = 'email'

If these are the same email tasks you are struggling with, then these two update queries will force them into a suspended state.  Once in a suspended state, you can just re-try them from Monitor page in LFForms.

UPDATE [LFForms].[dbo].[cf_bp_worker_instance_history]
SET [status] = 'suspended'
FROM [LFForms].[dbo].[cf_bp_worker_instances] AS wi
JOIN [LFForms].[dbo].[cf_bp_worker_instance_history] AS history ON wi.[instance_id] = history.[instance_id]
JOIN [LFForms].[dbo].[cf_bp_steps] AS step ON wi.[current_step_id] = step.[step_id] AND wi.[current_process_id] = step.[process_id]
WHERE wi.[status] = 3 AND history.[finish_date] IS NULL AND step.[step_sub_type] = 'email';

UPDATE [LFForms].[dbo].[cf_bp_worker_instances]
SET [status] = 11
FROM [LFForms].[dbo].[cf_bp_worker_instances] AS wi
JOIN [LFForms].[dbo].[cf_bp_worker_instance_history] AS history ON wi.[instance_id] = history.[instance_id]
JOIN [LFForms].[dbo].[cf_bp_steps] AS step ON wi.[current_step_id] = step.[step_id] AND wi.[current_process_id] = step.[process_id]
WHERE wi.[status] = 3 AND history.[finish_date] IS NULL AND step.[step_sub_type] = 'email';