OpenSMTPD with a single workstation

Hi :)

Is it necessary to use OpenSMTPD to receive and send emails via my ISP with a single machine as a workstation?

You do need something that can at least accept emails from localhost (such as cron emails) and either put them in a local mailbox or forward them to an upstream smtp server. Sendmail provided in base will do this, although OpenSMTPD is probably easier to configure.

If your ISP stores your email in a mailbox, which you access via POP3/IMAP/etc, you don't need an SMTP daemon for receiving email. If you have email delivered to you via SMTP (which is unlikely if you just have a workstation) you need something to receive those emails and put them in a mailbox. Again Sendmail can do this in base although it's perfectly reasonable to replace Sendmail with something else. I usually use Postfix, but that's probably overkill compared to OpenSMTPD if you just need something to handle local emails and submit them to your ISP.
If all you want is for your workstation to be able to accept and forward e-mails from localhost, you might also want to have a look at dma(8), which is included in base as of FreeBSD 11. Otherwise, if you want to be able to also receive e-mail from the Internet, then yes, you'll need something like OpenSMTPD.