What characters are illegal in password in ssmtp.conf?

There seem to be a number of outstanding bugs listed here with regards to problematic characters in the AuthPass directive:

• bug report 768129: the hash/pound character #
• bug report 463196: the equals = and the colon : characters

and I don't know if that would be the case for ssmtp specifically but generally bad characters to use in passwords are spaces, extended ASCII: è é ê ë etc. and UNICODE. Stick to a-z A-Z 0-9 .- and make passwords longer rather than more complex.