Commas are used in Thunderbird particularly, and under the bonnet more generally in email, as separators between multiple To: addresses. In Thunderbird, we generally recommend arranging one address per line and so by and large can avoid using them. If you really do need commas within an address, try wrapping it in inverted commas (quotation marks). I can't remember if commas are generally accepted as valid in email addresses. I need to look that up. I have met some cases of unlikely looking characters that are valid but not easy to get an email client to use. ISTR, for instance, the colon has a special use for some arcane group feature in email addressing.