please, create a new thread and there give us the name of your E-mail provider. If it is a private company account, you can send me a private message (via my profile here). Then, I have a look if I see something on the first glance. Because receiving works now, the issue must be something else than SHA-2. Therefore, that new thread (not to bloat this one here).
@smf1 same for you, please: We need the server address(es) of your E-mail provider. Here with my Symbian/S60 devices, I do no have any issue with 2048 bit keys/certificates. Therefore, I doubt this is the cause. If you have problems not just with E-mail (IMAP/SMTP) but Web (HTTPs) as well, we need at least one concrete example webpage to investigate.