diff --git a/EnvelopeGenerator.Application/Configurations/MailParams.cs b/EnvelopeGenerator.Application/Configurations/MailParams.cs
index 41d16e52..fb5d599b 100644
--- a/EnvelopeGenerator.Application/Configurations/MailParams.cs
+++ b/EnvelopeGenerator.Application/Configurations/MailParams.cs
@@ -1,4 +1,6 @@
-namespace EnvelopeGenerator.Application.Configurations;
+using System.Collections.Immutable;
+
+namespace EnvelopeGenerator.Application.Configurations;
 
 /// <summary>
 /// 
@@ -8,5 +10,5 @@ public class MailParams
     /// <summary>
     /// 
     /// </summary>
-    public required Dictionary<string, string> Placeholders { get; init; }
+    public required ImmutableDictionary<string, string> Placeholders { get; init; }
 }
\ No newline at end of file
diff --git a/EnvelopeGenerator.Application/Services/EnvelopeMailService.cs b/EnvelopeGenerator.Application/Services/EnvelopeMailService.cs
index 076fd11e..65221788 100644
--- a/EnvelopeGenerator.Application/Services/EnvelopeMailService.cs
+++ b/EnvelopeGenerator.Application/Services/EnvelopeMailService.cs
@@ -44,11 +44,11 @@ private readonly IAuthenticator _authenticator;
     public EnvelopeMailService(IEmailOutRepository repository, IMapper mapper, IEmailTemplateService tempService, IEnvelopeReceiverService envelopeReceiverService, IOptions<DispatcherParams> dispatcherConfigOptions, IConfigService configService, IOptions<MailParams> mailConfig, IAuthenticator authenticator) : base(repository, mapper)
     {
         _tempService = tempService;
-	_envRcvService = envelopeReceiverService;
-	_dConfig = dispatcherConfigOptions.Value;
-	_configService = configService;
-	_placeholders = mailConfig.Value.Placeholders;
-	_authenticator = authenticator;
+	    _envRcvService = envelopeReceiverService;
+	    _dConfig = dispatcherConfigOptions.Value;
+	    _configService = configService;
+	    _placeholders = new Dictionary<string, string>(mailConfig.Value.Placeholders);
+        _authenticator = authenticator;
     }
 
 private async Task<Dictionary<string, string>> CreatePlaceholders(string? accessCode = null, EnvelopeReceiverDto? envelopeReceiverDto = null)