Right now, hyperlinks are black by default, and they just look like regular underlined text which is not accessibility-friendly.

Please follow the default hyperlink colour norms for hyperlinks in emails by default, which W3 Schools lists as:

"By default, a link will appear like this (in all browsers):

An unvisited link is underlined and blue

A visited link is underlined and purple

An active link is underlined and red"

If users then want to change the colour, then leave the option to do so then, but the default should be internet-standards and norms.