When folding a long comment in an email header containing exclusively unfoldable characters, the parenthesis would not be preserved. This could be used for injecting headers into email messages where addresses are user-controlled and not sanitized.
Exploitability
AV:NAC:LAT:PPR:HUI:PVulnerable System
VC:NVI:HVA:NSubsequent System
SC:NSI:NSA:N5.7/CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N