This pretty much the same question asked five years ago here and never answered.
I am composing messages in Thunderbird, which encodes math as MathML. If I send this someone using Thunderbird, Outlook, or a number of other clients, it renders fine. If I open a test message in the RoundCube webmail client, it renders fine. But if my correspondent opens it in GMail, he gets something that looks like LaTeX (ie, unrendered MathML). While I would expect this in a Chrome browser, which doesn't support MathML, the problem occurs in Firefox as well. We've tested his Firefox browser with the MathML torture test and it correctly renders the inputs. This suggests that regardless of browser capability, the GMail web client is somehow blocking MathML.
Is there any way to tell the GMail client to render MathML? If not, what alternative strategies are available for reading a GMail message with MathML properly rendered. Obviously one could install Thunderbird or another mail client and connect via IMAP, but I'd prefer not to go that route. Likewise, forwarding to a different account is not a desirable choice.