Issue title: Add option to specify SMTP for mail
 Issue body: It would be nice to add the "-S" option of mailx to be able to choose the smtp to send the email
for instance
-S smtp=smtp://smtp.ust.hk
 Comments: 
 Comment 0: I'm reluctant to add any additional mail processing logic to the script. You should be able to add site specific configuration options to your .mailrc. That seems like a better place for this.
{'source': 'GithubIssues', 'repo_name_with_owner': 'Matty9191/ssl-cert-check', 'issue_id': 382387752, 'repo_stars': 598.0}