Amazon's SES mail service requires DKIM authentication. One step of authentication is to add a CNAME record to your domain's DNS.
Unfortunately the CNAME record has an underscore. My registrar, Network Solutions, does not allow underscores in CNAME records.
Is there a workaround for this? Transferring to a different registrar is an option, but obviously a horrible option.
DKIM is done with TXT records. Of course you could have a CNAME record (or chain) that points to a TXT record, but it is much more common to just create a TXT record directly.
Your DNS authoritative nameservice provider should let you put labels with underscores (which DKIM requires) into your domain's zone file. If not, then select a different DNS nameservice provider or use your own nameservers.
What DNS registrar you are using has nothing to do with it. The registrar does not control the contents of the domain nor would they even be aware of it.
It may be that your registrar and DNS nameservice provider happen to be the same organization, but they are separate roles and should be considered separately.