When you need to use sendmail method to send your mails and modify VtigerCRM files to handle this, you will get issues in Workflow Designer, because our module bring his own library to send mails.

But Workflow Designer contains an easy to use way to switch from SMTP to Sendmail.

You only needs to add a “Sendmail” Mail provider within Provider Manager and select this provider within every “send mail” block.
Then Workflow Designer automatically use sendmail without any file modifications.