The report, Delivering for all, exposes barriers to accessing mail for some people, denying them access to important information and services from banks, utilities, lawyers and medical services.
Affected groups include asylum seekers, the homeless, people living in remote and rural areas, survivors of domestic violence and the Gypsy / traveler community.
Problems faced by these groups include intercepted mail, delayed delivery, lack of collection points and affordability.
The CAS said the reluctance of government agencies and organizations to accept alternative addresses where people do not have a permanent mailing address or have frequent changes of address was also “of great concern.”
The report says the pandemic has dramatically affected the way Scots access postal services, which has had further impacts on those who were already at risk of not receiving mail and parcels.
He pointed out that the pandemic had brought about major changes for many people without permanent housing.
Many have been moved to temporary accommodation without shared facilities, mainly guesthouses and hotels, to reduce the risk of transmission of the virus, making it difficult for them to receive mail.
Citizens Advice has helped nearly 100,000 Scots since the lockdown
Meanwhile, stakeholders who work with asylum seekers reported that many support or legal agencies used by their clients closed their offices during closures and mail was rarely collected.
This caused potential difficulties when the mail was urgent and the information it contained concerned requests for benefits, housing or asylum.
Likewise, survivors of domestic violence and those from Gypsy and Traveler communities encountered difficulties in getting mail delivered to another address.
CAS spokesperson Gillian Fyfe said: “Regardless of the advancements in digital communication, the post remains an essential service in all communities across Scotland.
“The barriers we identified include the fact that people who do not have a permanent address have to arrange for their mail to be delivered to other addresses.
“Many organizations refuse to send mail, which can be vital or urgent for these addresses, which can increase the chances of the mail going missing. All of this can harm these consumers, either financially, personally or in their ability to access support services. “
She added: “The findings relating to domestic violence survivors are particularly distressing, as domestic violence survivors may run the risk of perpetrators intercepting the post.
“A more flexible system allowing access to other pickup or collection points would provide more secure access to the post for this group.
“Restricted access to mail is a significant issue for all groups considered in this research. These are people who we should make sure they are supported and that they are instead left behind. “
The report made a series of recommendations, including the Royal Mail examining the affordability of its redirect services and allowing people to use pickup points in the community – and government agencies sending copies of important correspondence to a designated person or at a secure location.
A Royal Mail spokesperson said: “Royal Mail understands the vital role mail plays in keeping vulnerable communities connected. We work hard to make our services both affordable and accessible to everyone.