equal signs omitted in body of message?

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When sending an e-mail via Mutt that contained a URL containing equal
signs, my recipient received the e-mail with equal signs deleted; thus,
a line of the form:

   https://www.youtube.com/watch?v=vDktNSENjUU

appeared to the recipient as the unusable:

   https://www.youtube.com/watch?vvDktNSENjUU

and this is how the line appeared when my message was quoted in my
recipient's reply to me.  Following the discussion here:

   https://stackoverflow.com/questions/38324093/php-html-email-equals-sign-in-email-is-being-converted-to-different-characte

I attempted to resend the message with the original URL written as:

   https://www.youtube.com/watch?v=3DvDktNSENjUU

but this didn't render a proper URL either.

Might the issue be that I send messages as plain text but the recipient's
MUA reads them as HTML?  Is there some setting in .muttrc that I could
adjust to fix this issue from my end?

Best regards,
Greg Marks

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