The server said

app/src/main/java/eu/faircode/email/EmailService.java

@@ -352,6 +352,15 @@ public class EmailService implements AutoCloseable {
                 AuthState authState = OAuthRefresh(context, provider, password);
                 connect(host, port, auth, user, authState.getAccessToken(), factory);
                 return authState.jsonSerializeString();
+            } else if (purpose == PURPOSE_CHECK) {
+                String msg = ex.getMessage();
+                if (msg != null)
+                    msg = msg.trim();
+                if (TextUtils.isEmpty(msg))
+                    throw ex;
+                throw new AuthenticationFailedException(
+                        context.getString(R.string.title_service_auth, msg),
+                        ex.getNextException());
             } else
                 throw ex;
         } catch (MailConnectException ex) {

app/src/main/res/values/strings.xml

@@ -676,6 +676,7 @@
     <string name="title_oauth_support">OAuth is not supported</string>
     <string name="title_review">Review</string>
     <string name="title_hint_alias">Long press an identity to display options, like copy to create alias identities</string>
+    <string name="title_service_auth">The email server said: \'%1$s\'</string>
     <string name="title_service_vpn">This might be caused by the VPN that is being used</string>
     <string name="title_service_port">Please double check the port number</string>
     <string name="title_service_protocol">Please double check the protocol (SSL/TLS or STARTTLS)</string>