Comment by rmoriz

I had this issue with let's encrypt certificates. IMHO it's unreleated to the issue. IOS18 mail initially connected using TLSv1.3. Afer deleting and re-installing Mail, the connection falls back to TLSSv1.2 and another set of ciphers. Then downloading of mails works again.

Using dovecot 2.3/Ubuntu on the server.