Comparison to `lettre`

[mo8it]

Hi :)

Could you please compare mail-send to lettre (since lettre is older)?

It would be awesome to add that comparison to the README? I am sure it would help people looking for an email client :D

[mdecimus]

Hi,

Would you like to submit a PR with the comparison?
I think the main difference is that lettre does not support DKIM signing (not sure if this was developed recently) while mail-send does.
On the other hand mail-send is async only and I think that lettre is also sync.

[Congyuwang]

lettre has connection pooling. So you get reconnecting and connection reuse for free. But maybe just wrap it with deadpool is good enough.