email are often not sent (stay in outbox) with the error message: "One or more recipients are invalid"

I send a lot of emails. Once in a while an email gets stuck with this error message: “One or more recipients are invalid”. There is nothing wrong with the email adresses. Sometimes sending manually helps (i.e. opening the mail in the outbox and sending again), some times not, and I have to send via another program.

I am attaching the log and screenshot.

The log file (a second error since I just tried to add a public contacts folder in the hope it will solve the problem)
11:05:55 erubin@bgu.ac.il [Exchange Web Services]  Synchronizing folder list
11:05:55 erubin@bgu.ac.il [Exchange Web Services]  Sending email
11:05:55 erubin@bgu.ac.il [Exchange Web Services]  Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:05:55   MailClient.Storage.Application.OperationException: Sending email failed due to the following error: One or more recipients are invalid. —> Microsoft.Exchange.WebServices.Data.ServiceResponseException: One or more recipients are invalid.
11:05:55    at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()
11:05:55    at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute() 11:05:55    at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable1 items, FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode, ServiceErrorHandling errorHandling)
11:05:55    at Microsoft.Exchange.WebServices.Data.ExchangeService.CreateItem(Item item, FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode)
11:05:55    at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode)
11:05:55    at Microsoft.Exchange.WebServices.Data.EmailMessage.InternalSend(FolderId parentFolderId, MessageDisposition messageDisposition)
11:05:55    at MailClient.Protocols.Exchange.ExchangeMailSynchronizer.<>c__DisplayClass18_0.b__0(WorkerStatus status, CancellationToken cancellationToken)
11:05:55 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:05:55 — End of inner exception stack trace —
11:05:55 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:05:55 at MailClient.Commands.Command.Process(WorkerStatus status)
11:05:56 erubin@bgu.ac.il [Exchange Web Services] Synchronizing folder 'Public Folders/ספר כתובות פנימי/'
11:05:57 erubin@bgu.ac.il [Exchange Web Services] Synchronizing folder 'Public Folders/Health Sciences Contacts/'
11:05:57 MailClient.Storage.Application.OperationException: Synchronizing folder ‘Public Folders/Health Sciences Contacts/’ failed due to the following error: Access is denied. Check credentials and try again. —> Microsoft.Exchange.WebServices.Data.ServiceResponseException: Access is denied. Check credentials and try again.
11:05:57 at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()
11:05:57 at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute()<br>11:05:57 at MailClient.Protocols.Exchange.ExchangeItemSynchronizer2.Synchronize(IItemSynchronizeContext1 synchronizeContext, Folder folder, CancellationToken cancellationToken)<br>11:05:57 at MailClient.Protocols.Common.ItemSynchronizer2.<>c__DisplayClass32_0.b__1(WorkerStatus status, CancellationToken cancellationToken)
11:05:57 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:05:57 — End of inner exception stack trace —
11:05:57 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:05:57 at MailClient.Commands.Command.Process(WorkerStatus status)
11:06:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:07:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:08:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:09:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:10:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:11:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:12:08 erubin@bgu.ac.il [Exchange Web Services] Synchronizing folder list
11:12:08 erubin@bgu.ac.il [Exchange Web Services] Sending email
11:12:08 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:12:09 erubin@bgu.ac.il [Exchange Web Services] Synchronizing folder 'Public Folders/ספר כתובות פנימי/'
11:12:09 MailClient.Storage.Application.OperationException: Sending email failed due to the following error: One or more recipients are invalid. —> Microsoft.Exchange.WebServices.Data.ServiceResponseException: One or more recipients are invalid.
11:12:09 at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()
11:12:09 at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute()<br>11:12:09 at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable1 items, FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode, ServiceErrorHandling errorHandling)
11:12:09 at Microsoft.Exchange.WebServices.Data.ExchangeService.CreateItem(Item item, FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode)
11:12:09 at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode)
11:12:09 at Microsoft.Exchange.WebServices.Data.EmailMessage.InternalSend(FolderId parentFolderId, MessageDisposition messageDisposition)
11:12:09 at MailClient.Protocols.Exchange.ExchangeMailSynchronizer.<>c__DisplayClass18_0.b__0(WorkerStatus status, CancellationToken cancellationToken)
11:12:09 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:12:09 — End of inner exception stack trace —
11:12:09 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:12:09 at MailClient.Commands.Command.Process(WorkerStatus status)
11:12:09 erubin@bgu.ac.il [Exchange Web Services] Synchronizing folder 'Public Folders/Health Sciences Contacts/'
11:12:09 MailClient.Storage.Application.OperationException: Synchronizing folder ‘Public Folders/Health Sciences Contacts/’ failed due to the following error: Access is denied. Check credentials and try again. —> Microsoft.Exchange.WebServices.Data.ServiceResponseException: Access is denied. Check credentials and try again.
11:12:09 at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()
11:12:09 at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute()<br>11:12:09 at MailClient.Protocols.Exchange.ExchangeItemSynchronizer2.Synchronize(IItemSynchronizeContext1 synchronizeContext, Folder folder, CancellationToken cancellationToken)<br>11:12:09 at MailClient.Protocols.Common.ItemSynchronizer2.<>c__DisplayClass32_0.b__1(WorkerStatus status, CancellationToken cancellationToken)
11:12:09 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:12:09 — End of inner exception stack trace —
11:12:09 at MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)
11:12:09 at MailClient.Commands.Command.Process(WorkerStatus status)
11:12:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:13:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט
11:14:51 erubin@bgu.ac.il [Exchange Web Services] Sending email: צריך עזרה בכיסוי עלויות הפרויקט