Maybe there are some other users on here who can comment.
I thought it was working perfectly and like the look of it. However I have several email accounts from the same isp and tend to only use one of them for general use and restrict what “machines” the others go on when I send emails. The general use one was the first I installed into sylpheed and is OK. I can send and receive. I added one of the others and it receives ok but I can’t send. The server just times out. The settings are identical even when the config file is compared line by line so not sure if this is a feature of the package or some sort of problem with my isp.
I previously used kmail pim3 on kde4 and am pretty sure I could send with all addresses on that even if I changed the mail transport setting to match the email address I was using. It would also send if that was left at the default one for the one I use most. However the isp might have decided to block that sort of thing now - different email address in the header to the from field.Sylpheed appears to use the correct mail transport so they should match.