Self-hosting has a severe metadata tradeoff. Suppose that you and I each self-host a mail server. Now we e-mail each other to talk about how great decentralization is and how much we both hate wiretapping.
With self-hosting and SMTP, there is now a very clear TCP metadata record showing how my home IP address connected on port 25 to your home IP address. Maybe the next day you write back, and there's another nice clear metadata record showing your address connecting to me. Now any ISP between us, and anyone who can tap the ISPs' cables, knows that you and I are corresponding.
If we were both using Gmail, this metadata pattern would pretty much not appear at all: we would each make an HTTPS connection to Gmail and exchange a bunch of data, and while in principle my upload would be matched in size by your download, it would be extraordinarily noisy in many ways and hard to correlate in practice. On the other hand, Gmail would know everything about us.
Having Gmail know everything about us is clearly terrible and not a good privacy solution. However, having all of the ISPs be able to learn our detailed correspondence patterns and relationships is also clearly terrible and not a good privacy solution. So, self-hosting being a clear privacy win for messaging will require a lot of technical improvements on the metadata front. It isn't a clear win in this respect today; it most likely depends on whether you see ISPs (and people who can compromise or coerce them) or Google (and people who can compromise or coerce them) as a bigger risk overall.
With self-hosting and SMTP, there is now a very clear TCP metadata record showing how my home IP address connected on port 25 to your home IP address. Maybe the next day you write back, and there's another nice clear metadata record showing your address connecting to me. Now any ISP between us, and anyone who can tap the ISPs' cables, knows that you and I are corresponding.
If we were both using Gmail, this metadata pattern would pretty much not appear at all: we would each make an HTTPS connection to Gmail and exchange a bunch of data, and while in principle my upload would be matched in size by your download, it would be extraordinarily noisy in many ways and hard to correlate in practice. On the other hand, Gmail would know everything about us.
Having Gmail know everything about us is clearly terrible and not a good privacy solution. However, having all of the ISPs be able to learn our detailed correspondence patterns and relationships is also clearly terrible and not a good privacy solution. So, self-hosting being a clear privacy win for messaging will require a lot of technical improvements on the metadata front. It isn't a clear win in this respect today; it most likely depends on whether you see ISPs (and people who can compromise or coerce them) or Google (and people who can compromise or coerce them) as a bigger risk overall.