diff --git a/src/activitypub/receiver.rs b/src/activitypub/receiver.rs
index 233f717..627a4a5 100644
--- a/src/activitypub/receiver.rs
+++ b/src/activitypub/receiver.rs
@@ -9,7 +9,7 @@ use crate::config::{Config, Instance};
 use crate::database::{Pool, get_database_client};
 use crate::errors::{DatabaseError, HttpError, ValidationError};
 use crate::models::attachments::queries::create_attachment;
-use crate::models::posts::types::{Post, PostCreateData, Visibility};
+use crate::models::posts::mentions::mention_to_acct;
 use crate::models::posts::queries::{
     create_post,
     get_post_by_id,
@@ -17,6 +17,7 @@ use crate::models::posts::queries::{
     delete_post,
 };
 use crate::models::posts::tags::normalize_tag;
+use crate::models::posts::types::{Post, PostCreateData, Visibility};
 use crate::models::profiles::queries::{
     get_profile_by_actor_id,
     get_profile_by_acct,
@@ -241,13 +242,9 @@ pub async fn process_note(
                         tags.push(tag_name);
                     };
                 } else if tag.tag_type == MENTION {
-                    if let Some(href) = tag.href {
-                        let profile = get_or_fetch_profile_by_actor_id(
-                            db_client,
-                            &instance,
-                            &href,
-                            &config.media_dir(),
-                        ).await?;
+                    // Ignore invalid mentions
+                    if let Ok(acct) = mention_to_acct(&instance.host(), &tag.name) {
+                        let profile = get_profile_by_acct(db_client, &acct).await?;
                         if !mentions.contains(&profile.id) {
                             mentions.push(profile.id);
                         };
diff --git a/src/models/posts/mentions.rs b/src/models/posts/mentions.rs
index 1a40222..9bd3fbe 100644
--- a/src/models/posts/mentions.rs
+++ b/src/models/posts/mentions.rs
@@ -3,11 +3,12 @@ use std::collections::HashMap;
 use regex::{Captures, Regex};
 use tokio_postgres::GenericClient;
 
-use crate::errors::DatabaseError;
+use crate::errors::{DatabaseError, ValidationError};
 use crate::models::profiles::queries::get_profiles_by_accts;
 use crate::models::profiles::types::DbActorProfile;
 
-const MENTION_RE: &str = r"(?m)(?P<space>^|\s)@(?P<user>\w+)@(?P<instance>\S+)";
+const MENTION_RE: &str = r"@?(?P<user>\w+)@(?P<instance>.+)";
+const MENTION_SEARCH_RE: &str = r"(?m)(?P<space>^|\s)@(?P<user>\w+)@(?P<instance>\S+)";
 
 fn pattern_to_acct(caps: &Captures, instance_host: &str) -> String {
     if &caps["instance"] == instance_host {
@@ -22,7 +23,7 @@ fn find_mentions(
     instance_host: &str,
     text: &str,
 ) -> Vec<String> {
-    let mention_re = Regex::new(MENTION_RE).unwrap();
+    let mention_re = Regex::new(MENTION_SEARCH_RE).unwrap();
     let mut mentions = vec![];
     for caps in mention_re.captures_iter(text) {
         let acct = pattern_to_acct(&caps, instance_host);
@@ -53,12 +54,13 @@ pub fn replace_mentions(
     instance_url: &str,
     text: &str,
 ) -> String {
-    let mention_re = Regex::new(MENTION_RE).unwrap();
+    let mention_re = Regex::new(MENTION_SEARCH_RE).unwrap();
     let result = mention_re.replace_all(text, |caps: &Captures| {
         let acct = pattern_to_acct(caps, instance_host);
         match mention_map.get(&acct) {
             Some(profile) => {
-                // Replace with a link
+                // Replace with a link to profile.
+                // Actor URL may differ from actor ID.
                 let url = profile.actor_url(instance_url).unwrap();
                 format!(
                     // https://microformats.org/wiki/h-card
@@ -74,6 +76,17 @@ pub fn replace_mentions(
     result.to_string()
 }
 
+pub fn mention_to_acct(
+    instance_host: &str,
+    mention: &str,
+) -> Result<String, ValidationError> {
+    let mention_re = Regex::new(MENTION_RE).unwrap();
+    let mention_caps = mention_re.captures(mention)
+        .ok_or(ValidationError("invalid mention tag"))?;
+    let acct = pattern_to_acct(&mention_caps, instance_host);
+    Ok(acct)
+}
+
 #[cfg(test)]
 mod tests {
     use serde_json::json;
@@ -131,4 +144,14 @@ mod tests {
         );
         assert_eq!(result, expected_result);
     }
+
+    #[test]
+    fn test_mention_to_acct() {
+        let mention = "@user@example.com";
+        let acct_1 = mention_to_acct("example.com", mention).unwrap();
+        assert_eq!(acct_1, "user");
+
+        let acct_2 = mention_to_acct("server.info", mention).unwrap();
+        assert_eq!(acct_2, "user@example.com");
+    }
 }
diff --git a/src/models/posts/tags.rs b/src/models/posts/tags.rs
index 04f067b..05091ab 100644
--- a/src/models/posts/tags.rs
+++ b/src/models/posts/tags.rs
@@ -1,6 +1,6 @@
 use regex::{Captures, Regex};
 
-use crate::errors::ConversionError;
+use crate::errors::ValidationError;
 use crate::frontend::get_tag_page_url;
 
 const HASHTAG_RE: &str = r"(?m)(?P<before>^|\s)#(?P<tag>\S+)";
@@ -52,11 +52,11 @@ pub fn replace_tags(instance_url: &str, text: &str, tags: &[String]) -> String {
     result.to_string()
 }
 
-pub fn normalize_tag(tag: &str) -> Result<String, ConversionError> {
+pub fn normalize_tag(tag: &str) -> Result<String, ValidationError> {
     let hashtag_name_re = Regex::new(HASHTAG_NAME_RE).unwrap();
     let tag_name = tag.trim_start_matches('#');
     if !hashtag_name_re.is_match(tag_name) {
-        return Err(ConversionError);
+        return Err(ValidationError("invalid tag name"));
     };
     Ok(tag_name.to_lowercase())
 }