nostr:npub1xa7jr5wl773mwvuduve29zj8n53vfgcqlulaq0mma85xr75hwv9svzy7yc nostr:npub10kpejapvt6u8m7ls45aspdg9935v50fqegm6fq4ex8n2v3ajavgqz80rjj because Thunderbird and some other clients gave in and implemented special features to support GMail (namely, OAuth authentication). None of the older mail clients will work with GMail today.

Also, Google manually approves a list of mail clients you are allowed to use with your GMail account. For example, check out these notes from Pegasus Mail developer.

https://www.pmail.com/newsflash.htm