Yes, SMTP was designed in the days of much less reliable internet connections (either systems that only had connectivity at intermittent times, or just basic unreliability). The protocol has a built in assumption that a receiver might not be accessible "now" and so servers are required (in order to be compliant to the standard) to hold, and retry, over several days, to deliver an email before giving up and returning it to the sender.
So it is quite feasible to run email over dialup. You just have to periodically connect, and remain connected for a reasonable period of time, and SMTP will work just fine (only with higher latency than we are accustomed to in 2019).
When I was a teen, I ran a personal mail server on a dynamic IP (using DynDNS) over dialup with about 25% uptime. Miraculously, mail still got delivered to my server. I was in awe of the reliability of SMTP.
The greatest problem today affecting reliablity is that mail gets dropped silently by the major providers.