Yeah. I use a Raspberry 2 for my mail server, but I use an SSD in a USB enclosure for mail spool. It's also faster, but above all, it's more robust than SD cards.

USB on all Raspberry Pi below 4 is capped at 480 mbit/sec due to it being USB2. Pi 4 has USB3, but it can interfere with 2.4 GHz signals.

USB2 is still faster than all but V90 class MicroSD cards. And V90 cards are only rated for 90MB/s writes, not reads.

Even if you had a UHS-III V90 A2 card, a USB2 SSD is going to give you 10x the IOP/s performance.

Even the USB2 speeds are overkill for most server stuff, where you're more commonly making a bunch of small random accesses than long streaming reads or writes. Not that you can't, but most people are going to be running things like Wordpress or other relatively lightweight services on an RPi.

yes, the microSD card isn't known for being super speedy (but it's not absolutely terrible either).

mSD is slower than an SSD.