There are still a number of outstanding bugs dealing with special characters on the AuthPass line in the /etc/ssmtp/ssmtp.conf file:
https://bugs.debian.org/cgi-bin/pkgreport.cgi?repeatmerged=no&src=ssmtp#_0_3_4
These bug reports are specifically for the ssmtp page in Debian, but probably apply to others.
- Debian Bug report logs - #463196: ssmtp cannot parse AuthPass with '=' or ':' in it
- Debian Bug report logs - #768129: ssmtp: Same problem here with "#"
According to these report logs, lines containing the characters =, :, or # are either skipped entirely, or processed in a way that removes the part of password on the right-hand side these characters.
In other words, there is currently no way to "escape" these special characters on the AuthPass line.
As a side note, there are a number of devices, such as wireless home printers, that require passwords to be entered on a number pad, similar to those used on telephones. Some special characters are available on these devices, but many cannot be selected.
For situations like this, you are (unfortunately) forced to adjust your password for the device.
A longer password consisting of letters & numbers is preferred to a shorter one with more punctuation characters.
This question has also been asked and answered very well on the following page:
What characters are illegal in password in ssmtp.conf?