Albeit in Windows, we're still using Pegasus Mail, set to show messages in plain text by default. Fair point about clicking on links, but our setting must be the most reliable way to block viruses, as the client doesn't execute anything, malicious or not.
Pegasus is not a candidate on Linux, but I'm not sure I agree about "functionally limited" clients being that much of an issue.
I find the modern email world a bit perverse. BigCorp, who account for loads of all our emails, send out really fancy messages with loads of images and stuff. But BigCorp's IT department block images and code in messages received by their staff. BigCorp's marketing team send most of their stuff to potential clients in LargerFirm, whose marketing and IT teams do the same in return. So who exactly is seeing all these fancy HTML messages?
For example, Mrs Drossall and I, working for different organisations, get the same staff incentives newsletters, because both employers use the same supplier. Neither of us can see what incentives are being offered to us, since both our IT departments block images, and the supplier always puts the vital text in the pictures