Dreams involving technology, relationships, and past events can often reflect our current feelings, unresolved issues, or desires. In your dream, several key elements stand out:

1. The Office Setting: This may symbolize your professional life or personal identity. It could indicate feelings related to your work or how you perceive yourself in a professional environment.
2. Your Son's Tablet: Using your son's tablet might represent your connection to family or your feelings towards your son. The fact that it had a fabric cover resembling a cloud or pillow suggests a sense of comfort or protection, perhaps indicating your desire to shield your son from the complexities of adult relationships.
3. The Photo of Your Ex-Husband and His Mistress: This image could symbolize lingering feelings about your past relationship and the emotions tied to it. Seeing them in a mundane setting like grocery shopping may indicate that you are processing the normalcy of their relationship, which can be painful.
4. Accessing His Gmail Account: Logging into your ex-husband's email suggests a desire to uncover the truth or understand something about your past. This action can signify feelings of betrayal, curiosity, or unresolved emotions regarding your divorce and his new relationship.
5. The "Her" Folder and Emails: The folder labeled "Her" may represent your feelings of being replaced or forgotten. The content of the emails suggests a juxtaposition between your ex-husband moving on and your own feelings of loss or confusion. The way the mistress writes, described as almost unintelligible, might reflect your perception of their relationship as immature or lacking depth.

Overall, this dream could be a manifestation of your subconscious processing your past relationship, feelings of betrayal, and concerns about your son’s connection to your ex-husband. It may be helpful to reflect on how these feelings impact your current life and relationships. Consider whether there are unresolved emotions you need to address or if it’s time to focus on your own healing and future.