First, wonder if the moderator of this forum might consider moving your question to a thread of it's own, rather than piggy-back on Cron issue.
It's been my experience, that when one uses Linux (pick your distro) and elects to run a web server + options, what gets installed is the AMP stack but also, either sendmail (the granddaddy of them all) or postfix or mail server option of your choice (Exim, other).
Sendmail, for example, is really two parts ... the sending part is the MTA and apache will use as defined in your php.ini. It can send mail out but not configured to receive mail other than from itself (localhost). And that's all you really want Moodle to be able to do ... send the mail (not receive it like a full blown mail server).
As far as your Moodle not having issues with the sending of mail without configuring any SMTP server, sendmail/postfix itself could be configured to send all mail via an SMTP server. Hosting providers would more than likely confiigure the packages rented to customers like that.
A standalone server ... one installed via net or CD/DVD from scratch (the kind I usually do/have done) requires one to install other than the default MTA ... CentOS 5 and below pre-installed sendmail. CentOS 6/7 has changed to Postfix. It's up to the OP, however, to configure it as per 'taste'. Me ... I have some boxen receiving mail but only from restricted IP addresses/FQDN's. Would not recommend that for most Moodle server implementations as it's one more thing to configure correctly/protect and maintain. Keeping up with the "Moodle March" is enough for most folks ... including me! ;)
Realize this is not a really technical explanation, but hope it's enough to answer the question (given the information shared).
'spirit of sharing', Ken