Moving large system directories to another drive and using symlink?
I would recommend using mountpoints instead of symbolic links, but symbolic links will work.

For example, I have a small SSD as a boot drive and a PCIe SSD for my /Users partition on my Mac Pro.

I copied over the data and added the following to my /etc/fstab file.

LABEL=Users /Users  hfs rw,auto


"Users" is the name of the partition known to Disk Utility and "/Users" is the mount point.

This is a lot cleaner than symbolic links, as many tools will not follow symbolic links without being told explicitly to do so, to avoid cyclic links.

In your case, you might want to create a separate partition for ~/Library and move everything over.  You may also want to create separate partitions for Mail and for MobileSync.

All of that said, symlinks will very likely work in these two cases.  However, keep in mind that you're moving directories that have both a high rate of change and a lot of data, which is the worst case for a spindle.  Expect iOS backups to be slower and for Mail operations such as "search" and opening attachments to take a little longer.