Sylphee cannot send

Hi, since about two weeks after a large block of updates, suddenly I cannot send messages with Sylpheed, I do get an error message that suggest my E-mail provider blocked me. Error code: 554 5.7.1
I checked with Startmail, My ISP Summit Broadband but everything checks out OK.
I can send messages through Sylpheed with G Mail.
Also, it takes a very long time for Sylpheed to open up when I click on the icon…45 sec to a minute.
I do receive messages with Sylphheed/Startmail and everything else is normal, I just can’t send.

Anybody out there has the same problem?

Did you try uninstalling Sylpheed and then reinstalling it?

emerge -avC Sylpheed
emerge -av Sylpheed