I cannot send e-mails - mail server offline??

I cannot send e-mails and investigated it through the Mail Preferences, clicked on accounts and noticed my outgoing mail server (SMTP) is offline (mail.optonline.net). How did this happen and how do I get the outgoing mail server to come back on line????? My e-mail address looks like the following: ##########@optonline.net PLEASE HELP!

When I send an e-mail, I get the following message:

Cannot send message using the server mail.optonline.net

The SMTP connection to server "mail.optonline.net" failed.

Select a different outgoing mail server from the list below or click Try Later to leave message in your Outbox until it can be sent.

The options are: Edit SMTP Server List / Connection Doctor / Edit Message / Try Later / Try with Selected Server

Thank you so much for your help in advance!

Sue

iMac (21.5-inch, Late 2013), OS X El Capitan (10.11.3)