When sending a few consecutive messages to a friend, it seems that riseup blocks sending new messages for about 10 minutes.
When accessing the email and trying to send something, I get blocked too.
Oh I wonder, does the same thing happen on other servers?
It is unfeasible to use delta as a main chat
In all likelihood the problem is related to the low rate limit set by the provider for sending e-mail (this is usually done to prevent the service from being used to send spam), unfortunately the only solutions I can think of are:
contact the provider and ask for a suspension of the rate limit for the account explaining well the reasons for the request, hoping that the provider will be sympathetic
change provider to use with delta chat